Lithium-ion battery

A lithium-ion battery or Li-ion battery is a type of rechargeable battery that stores energy by moving lithium ions between special materials. These batteries are very powerful for their size, last a long time, and can be recharged many times. Since they were first sold in 1991, they have become much better and cheaper.

Lithium-ion batteries have changed our world. They power many things we use every day, like portable consumer electronics, laptop computers, cellular phones, and electric cars. They are also used to store energy for the power grid and in special machines for the military and space travel.

These batteries were developed over many years by scientists who received the 2019 Nobel Prize in Chemistry for their work. While they are very useful, they can sometimes be dangerous because they contain materials that can catch fire. Scientists are working on safer designs, like batteries that do not use flammable materials. There are also efforts to make these batteries better for the environment and to find new types that do not need as many special minerals.

History

Main article: History of the lithium-ion battery

Early research into lithium-ion batteries began in 1965 with a battery developed by NASA. A big step forward happened in 1974 when a scientist named M. Stanley Whittingham used a special material that could hold lithium ions. However, these early batteries had problems, like catching fire, so they were not made for sale.

In the 1980s, scientists found better materials that were safer and worked well. They began using a different material for the battery’s parts, which helped make the batteries safer. In 1991, a company named Sony started selling the world’s first lithium-ion batteries for people to use. Since then, scientists have kept improving these batteries, making them better and more powerful. Today, lithium-ion batteries are used in many things, like phones and laptops, because they store a lot of energy in a small space.

Design

Generally, the negative electrode of a conventional lithium-ion cell is made from graphite. The positive electrode is typically a metal oxide or phosphate. The electrolyte is a lithium salt in an organic solvent. The negative electrode (which is the anode when the cell is discharging) and the positive electrode (which is the cathode when discharging) are prevented from shorting by a separator. The electrodes are connected to the powered circuit through two pieces of metal called current collectors.

The negative and positive electrodes swap their electrochemical roles (anode and cathode) when the cell is charged. Despite this, in discussions of battery design the negative electrode of a rechargeable cell is often just called "the anode" and the positive electrode "the cathode".

In its fully lithiated state of LiC₆, graphite correlates to a theoretical capacity of 1339 coulombs per gram (372 mAh/g). The positive electrode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide). More experimental materials include graphene-containing electrodes, although these remain far from commercially viable due to their high cost.

Lithium reacts vigorously with water to form lithium hydroxide (LiOH) and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes moisture from the battery pack. The non-aqueous electrolyte is typically a mixture of organic carbonates such as ethylene carbonate and propylene carbonate containing complexes of lithium ions. Ethylene carbonate is essential for making solid electrolyte interphase on the carbon anode, but since it is solid at room temperature, a liquid solvent (such as propylene carbonate or diethyl carbonate) is added.

The electrolyte salt is almost always^{[citation needed]} lithium hexafluorophosphate (LiPF₆), which combines good ionic conductivity with chemical and electrochemical stability. The hexafluorophosphate anion is essential for passivating the aluminium current collector used for the positive electrode. A titanium tab is ultrasonically welded to the aluminium current collector. Other salts like lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), and lithium bis(trifluoromethanesulfonyl)imide (LiC₂F₆NO₄S₂) are frequently used in research in tab-less coin cells, but are not usable in larger format cells, often because they are not compatible with the aluminium current collector. Copper (with a spot-welded nickel tab) is used as the current collector at the negative electrode.

Current collector design and surface treatments may take various forms: foil, mesh, foam (dealloyed), etched (wholly or selectively), and coated (with various materials) to improve electrical characteristics.

Depending on materials choices, the voltage, energy density, life, and safety of a lithium-ion cell can change dramatically. Current effort has been exploring the use of novel architectures using nanotechnology to improve performance. Areas of interest include nano-scale electrode materials and alternative electrode structures.

Electrochemistry

The reactants in the electrochemical reactions in a lithium-ion cell are the materials of the electrodes, both of which are compounds containing lithium atoms. Although many thousands of different materials have been investigated for use in lithium-ion batteries, only a very small number are commercially usable. All commercial Li-ion cells use intercalation compounds as active materials. The negative electrode is usually graphite, although silicon is often mixed in to increase the capacity. The electrolyte is usually lithium hexafluorophosphate, dissolved in a mixture of organic carbonates. A number of different materials are used for the positive electrode, such as LiCoO₂, LiFePO₄, and lithium nickel manganese cobalt oxides.

During cell discharge the negative electrode is the anode and the positive electrode the cathode: electrons flow from the anode to the cathode through the external circuit. An oxidation half-reaction at the anode produces positively charged lithium ions and negatively charged electrons. The oxidation half-reaction may also produce uncharged material that remains at the anode. Lithium ions move through the electrolyte; electrons move through the external circuit toward the cathode where they recombine with the cathode material in a reduction half-reaction. The electrolyte provides a conductive medium for lithium ions but does not partake in the electrochemical reaction. The reactions during discharge lower the chemical potential of the cell, so discharging transfers energy from the cell to wherever the electric current dissipates its energy, mostly in the external circuit.

During charging these reactions and transports go in the opposite direction: electrons move from the positive electrode to the negative electrode through the external circuit. To charge the cell the external circuit has to provide electrical energy. This energy is then stored as chemical energy in the cell (with some loss, e. g., due to coulombic efficiency lower than 1).

Both electrodes allow lithium ions to move in and out of their structures with a process called insertion (intercalation) or extraction (deintercalation), respectively.

As the lithium ions "rock" back and forth between the two electrodes, these batteries are also known as "rocking-chair batteries" or "swing batteries" (a term given by some European industries).

The following equations exemplify the chemistry (left to right: discharging, right to left: charging).

The negative electrode half-reaction for the graphite is

LiC₆ ⇌ C₆ + Li⁺ + e⁻

The positive electrode half-reaction in the lithium-doped cobalt oxide substrate is

CoO₂ + Li⁺ + e⁻ ⇌ LiCoO₂

The full reaction being

LiC₆ + CoO₂ ⇌ C₆ + LiCoO₂

The overall reaction has its limits. Overdischarging supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction:

Li⁺ + e⁻ + LiCoO₂ ⟶ Li₂O + CoO

Overcharging up to 5.2 volts leads to the synthesis of cobalt (IV) oxide, as evidenced by x-ray diffraction:

LiCoO₂ ⟶ Li⁺ + CoO₂ + e⁻

The cell's energy is equal to the voltage times the charge. Each gram of lithium represents Faraday's constant/6.941, or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kilogram of lithium. This is slightly more than the heat of combustion of gasoline; however, lithium-ion batteries as a whole are still significantly heavier per unit of energy due to the additional materials used in production.

Note that the cell voltages involved in these reactions are larger than the potential at which an aqueous solutions would electrolyze.

Discharging and charging

During discharge, lithium ions (Li⁺) carry the current within the battery cell from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm.

During charging, an external electrical power source applies an over-voltage (a voltage greater than the cell's own voltage) to the cell, forcing electrons to flow from the positive to the negative electrode. The lithium ions also migrate (through the electrolyte) from the positive to the negative electrode where they become embedded in the porous electrode material in a process known as intercalation.

Energy losses arising from electrical contact resistance at interfaces between electrode layers and at contacts with current collectors can be as high as 20% of the entire energy flow of batteries under typical operating conditions.

The charging procedures for single Li-ion cells, and complete Li-ion batteries, are slightly different:

• A single Li-ion cell is charged in two stages:

1. Constant current (CC)
2. Constant voltage (CV)

• A Li-ion battery (a set of Li-ion cells in series) is charged in three stages:

1. Constant current
2. Balance (only required when cell groups become unbalanced during use)
3. Constant voltage

During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the top-of-charge voltage limit per cell is reached.

During the balance phase, the charger/battery reduces the charging current (or cycles the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit until the battery is balanced. Balancing typically occurs whenever one or more cells reach their top-of-charge voltage before the other(s), as it is generally inaccurate to do so at other stages of the charge cycle. This is most commonly done by passive balancing, which dissipates excess charge as heat via resistors connected momentarily across the cells to be balanced. Active balancing is less common, more expensive, but more efficient, returning excess energy to other cells (or the entire pack) via a DC-DC converter or other circuitry. Balancing most often occurs during the constant voltage stage of charging, switching between charge modes until complete. The pack is usually fully charged only when balancing is complete, as even a single cell group lower in charge than the rest will limit the entire battery's usable capacity to that of its own. Balancing can last hours or even days, depending on the magnitude of the imbalance in the battery.

During the constant voltage phase, the charger applies a voltage equal to the maximum cell voltage times the number of cells in series to the battery, as the current gradually declines towards 0, until the current is below a set threshold of about 3% of initial constant charge current.

Periodic topping charge about once per 500 hours. Top charging is recommended to be initiated when voltage goes below 4.05 V/cell.

Failure to follow current and voltage limitations can result in excessive coulombic heating of the battery, and in the case of overcharge to voltages higher than designed can lead to an explosion.

Charging temperature limits for Li-ion are stricter than the operating limits. Lithium-ion chemistry performs well at elevated temperatures but prolonged exposure to heat reduces battery life. Li‑ion batteries offer good charging performance at cooler temperatures and may even allow "fast-charging" within a temperature range of 5 to 45 °C (41 to 113 °F). Charging should be performed within this temperature range. At temperatures from 0 to 5 °C charging is possible, but the charge current should be reduced. During a low-temperature (under 0 °C) charge, the slight temperature rise above ambient due to the internal cell resistance is beneficial. High temperatures during charging may lead to battery degradation and charging at temperatures above 45 °C will degrade battery performance, whereas at lower temperatures the internal resistance of the battery may increase, resulting in slower charging and thus longer charging times.

Batteries gradually self-discharge even if not connected and delivering current. Li-ion rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5–2% per month.

The rate increases with temperature and state of charge. A 2004 study found that for most cycling conditions self-discharge was primarily time-dependent; however, after several months of stand on open circuit or float charge, state-of-charge dependent losses became significant. The self-discharge rate did not increase monotonically with state-of-charge, but dropped somewhat at intermediate states of charge. Self-discharge rates may increase as batteries age. In 1999, self-discharge per month was measured at 8% at 21 °C, 15% at 40 °C, 31% at 60 °C. By 2007, monthly self-discharge rate was estimated at 2% to 3%, and 2–3% by 2016.

By comparison, the self-discharge rate for NiMH batteries dropped, as of 2017, from up to 30% per month for previously common cells to about 0.08–0.33% per month for low self-discharge NiMH batteries, and is about 10% per month in NiCd batteries.

Cathode

Transition metal oxides (TMOs) are widely used as cathode materials in lithium-ion batteries as the variable oxidation state of transition metal cations allows oxides of these metals to reversibly host lithium ions (Li⁺) and undergo efficient redox (reduction-oxidation) reactions. While Oxygen ions are commonly assumed to remain in a 2- oxidation state, the role of oxygen redox in facilitating the lithium insertion is now recognized as instrumental in the performance of lithium ion battery cathodes. The layered or framework structures of TMOs allow Li⁺ insertion/extraction during charging/discharging, while their transition metals and oxygen anions participate in electron transfer, enabling high energy density and stability. Three classes of cathode materials in lithium-ion batteries have been commercialized: (1) layered oxides, (2) spinel oxides and (3) oxoanion complexes. All of them were discovered by John Goodenough and his collaborators.

Layered oxides

LiCoO₂ was used in the first commercial lithium-ion battery made by Sony in 1991. The layered oxides have a pseudo-tetrahedral structure comprising layers made of MO₆ octahedra separated by interlayer spaces that allow for two-dimensional lithium-ion diffusion.^{[citation needed]} The band structure of Li_xCoO₂ allows for true electronic (rather than polaronic) conductivity. However, due to an overlap between the Co⁴⁺ t_2g d-band with the O^2- 2p-band, the x must be >0.5, otherwise O₂ evolution occurs. This limits the charge capacity of this material to ~140 mA h g⁻¹.

Several other first-row (3d) transition metals also form layered LiMO₂ salts. Some can be directly prepared from lithium oxide and M₂O₃ (e.g. for M=Ti, V, Cr, Co, Ni), while others (M= Mn or Fe) can be prepared by ion exchange from NaMO₂. LiVO₂, LiMnO₂ and LiFeO₂ suffer from structural instabilities (including mixing between M and Li sites) due to a low energy difference between octahedral and tetrahedral environments for the metal ion M. For this reason, they are not used in lithium-ion batteries. However, Na⁺ and Fe³⁺ have sufficiently different sizes that NaFeO₂ can be used in sodium-ion batteries.

Similarly, LiCrO₂ shows reversible lithium (de)intercalation around 3.2 V with 170–270 mAh/g. However, its cycle life is short, because of disproportionation of Cr⁴⁺ followed by translocation of Cr⁶⁺ into tetrahedral sites. On the other hand, NaCrO₂ shows a much better cycling stability. LiTiO₂ shows Li+ (de)intercalation at a voltage of ~1.5 V, which is too low for a cathode material.

These problems leave LiCoO₂ and LiNiO₂ as the only practical layered oxide materials for lithium-ion battery cathodes. The cobalt-based cathodes show high theoretical specific (per-mass) charge capacity, high volumetric capacity, low self-discharge, high discharge voltage, and good cycling performance. Unfortunately, they suffer from a high cost of the material. For this reason, the current trend among lithium-ion battery manufacturers is to switch to cathodes with higher Ni content and lower Co content.

In addition to a lower (than cobalt) cost, nickel-oxide based materials benefit from the two-electron redox chemistry of Ni: in layered oxides comprising nickel (such as nickel-cobalt-manganese NCM and nickel-cobalt-aluminium oxides NCA), Ni cycles between the oxidation states +2 and +4 (in one step between +3.5 and +4.3 V), cobalt- between +2 and +3, while Mn (usually >20%) and Al (typically, only 5% is needed) remain in +4 and 3+, respectively. Thus increasing the Ni content increases the cyclable charge. For example, NCM111 shows 160 mAh/g, while LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) and LiNi_0.8Co_0.15Al_0.05O₂ (NCA) deliver a higher capacity of ~200 mAh/g. NCM and NCA batteries are collectively called ternary lithium batteries.

It is worth mentioning so-called "lithium-rich" cathodes that can be produced from traditional NCM (LiMO₂, where M=Ni, Co, Mn) layered cathode materials upon cycling them to voltages/charges corresponding to Li:M2O₄ adopts a cubic lattice, which allows for three-dimensional lithium-ion diffusion. Manganese cathodes are attractive because manganese is less expensive than cobalt or nickel. The operating voltage of Li-LiMn₂O₄ battery is 4 V, and ca. one lithium per two Mn ions can be reversibly extracted from the tetrahedral sites, resulting in a practical capacity of 3+ is not a stable oxidation state, as it tends to disproportionate into insoluble Mn⁴⁺ and soluble Mn²⁺. LiMn₂O₄ can also intercalate more than 0.5 Li per Mn at a lower voltage around +3.0 V. However, this results in an irreversible phase transition due to Jahn-Teller distortion in Mn3+:t2g3eg1, as well as disproportionation and dissolution of Mn³⁺.

An important improvement of Mn spinel are related cubic structures of the LiMn_1.5Ni_0.5O₄ type, where Mn exists as Mn4+ and Ni cycles reversibly between the oxidation states +2 and +4. This materials show a reversible Li-ion capacity of ca. 135 mAh/g around 4.7 V. Although such high voltage is beneficial for increasing the specific energy of batteries, the adoption of such materials is currently hindered by the lack of suitable high-voltage electrolytes. In general, materials with a high nickel content are favored in 2023, because of the possibility of a 2-electron cycling of Ni between the oxidation states +2 and +4.

LiV₂O₄ (lithium vanadium oxide) operates as a lower (ca. +3.0 V) voltage than LiMn₂O₄, suffers from similar durability issues, is more expensive, and thus is not considered of practical interest.

Oxoanionic

Around 1980 Manthiram discovered that oxoanions (molybdates and tungstates in that particular case) cause a substantial positive shift in the redox potential of the metal-ion compared to oxides. In addition, these oxoanionic cathode materials offer better stability/safety than the corresponding oxides. However, they also suffer from poor electronic conductivity due to the long distance between redox-active metal centers, which slows down the electron transport. This necessitates the use of small (less than 200 nm) cathode particles and coating each particle with a layer of electronically-conducting carbon. This reduces the packing density of these materials.

Although numerous combinations of oxoanions (sulfate, phosphate, silicate) with various metals (mostly Mn, Fe, Co, Ni) have been studied, LiFePO₄ is the only one that has been commercialized. Although it was originally used primarily for stationary energy storage due to its lower energy density compared to layered oxides, it has begun to be widely used in electric vehicles since the 2020s.

Anode

Main article: Research in lithium-ion batteries § Anode

Negative electrode materials are traditionally constructed from graphite and other carbon materials, although newer silicon-based materials are being increasingly used (see Nanowire battery). In 2016, 89% of lithium-ion batteries contained graphite (43% artificial and 46% natural), 7% contained amorphous carbon (either soft carbon or hard carbon), 2% contained lithium titanate (LTO) and 2% contained silicon or tin-based materials.

These materials are used because they are abundant, electrically conducting and can intercalate lithium ions to store electrical charge with modest volume expansion (~10%). Graphite is the dominant material because of its low intercalation voltage and excellent performance. Various alternative materials with higher capacities have been proposed, but they usually have higher voltages, which reduces energy density. Low voltage is the key requirement for anodes; otherwise, the excess capacity is useless in terms of energy density.

As graphite is limited to a maximum capacity of 372 mAh/g much research has been dedicated to the development of materials that exhibit higher theoretical capacities and overcoming the technical challenges that presently encumber their implementation. The extensive 2007 Review Article by Kasavajjula et al. summarizes early research on silicon-based anodes for lithium-ion secondary cells. In particular, Hong Li et al. showed in 2000 that the electrochemical insertion of lithium ions in silicon nanoparticles and silicon nanowires leads to the formation of an amorphous Li–Si alloy. The same year, Bo Gao and his doctoral advisor, Professor Otto Zhou described the cycling of electrochemical cells with anodes comprising silicon nanowires, with a reversible capacity ranging from at least approximately 900 to 1500 mAh/g.

Diamond-like carbon coatings can increase retention capacity by 40% and cycle life by 400% for lithium based batteries.

To improve the stability of the lithium anode, several approaches to installing a protective layer have been suggested. Silicon is beginning to be looked at as an anode material because it can accommodate significantly more lithium ions, storing up to 10 times the electric charge, however this alloying between lithium and silicon results in significant volume expansion (ca. 400%), which causes catastrophic failure for the cell. Silicon has been used as an anode material but the insertion and extraction of Li + {\displaystyle {\ce {\scriptstyle Li+}}} !{\displaystyle {\scriptstyle \mathrm {Li} {\vphantom {A}}^{+}}} can create cracks in the material. These cracks expose the Si surface to an electrolyte, causing decomposition and the formation of a solid electrolyte interphase (SEI) on the new Si surface (crumpled graphene encapsulated Si nanoparticles). This SEI will continue to grow thicker, deplete the available Li + {\displaystyle {\ce {\scriptstyle Li+}}} !{\displaystyle {\scriptstyle \mathrm {Li} {\vphantom {A}}^{+}}} , and degrade the capacity and cycling stability of the anode.

In addition to carbon- and silicon- based anode materials for lithium-ion batteries, high-entropy metal oxide materials are being developed. These conversion (rather than intercalation) materials comprise an alloy (or subnanometer mixed phases) of several metal oxides performing different functions. For example, Zn and Co can act as electroactive charge-storing species, Cu can provide an electronically conducting support phase and MgO can prevent pulverization.

Electrolyte

Liquid electrolytes in lithium-ion batteries consist of lithium salts, such as LiPF
₆, LiBF
₄, LiFSI, LiTFSI or LiClO
₄ in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate. A liquid electrolyte acts as a conductive pathway for the movement of cations passing from the negative to the positive electrodes during discharge. Typical conductivities of liquid electrolyte at room temperature (20 °C (68 °F)) are in the range of 10 mS/cm, increasing by approximately 30–40% at 40 °C (104 °F) and decreasing slightly at 0 °C (32 °F). The combination of linear and cyclic carbonates (e.g., ethylene carbonate (EC) and dimethyl carbonate (DMC)) offers high conductivity and solid electrolyte interphase (SEI)-forming ability. While EC forms a stable SEI, it is not a liquid at room temperature, only becoming a liquid with the addition of additives such as the previously mentioned DMC or diethyl carbonate (DEC) or ethyl methyl carbonate (EMC). Organic solvents easily decompose on the negative electrodes during charge. When appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial charging and forms a solid layer called the solid electrolyte interphase, which is electrically insulating, yet provides significant ionic conductivity, behaving as a solid electrolyte. The interphase prevents further decomposition of the electrolyte after the second charge as it grows thick enough to prevent electron tunneling after the first charge cycle. For example, ethylene carbonate is decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable interface. Composite electrolytes based on POE (poly(oxyethylene)) provide a relatively stable interface. It can be either solid (high molecular weight) and be applied in dry Li-polymer cells, or liquid (low molecular weight) and be applied in regular Li-ion cells. Room-temperature ionic liquids (RTILs) are another approach to limiting the flammability and volatility of organic electrolytes.

Solid electrolyte interphase (SEI)

The term solid electrolyte interphase was first coined by Peled in 1979 to describe the layer of insoluble products deposited on alkali and alkaline earth cathodes in non-aqueous batteries (NAB). However, Dey and Sullivan had noted previously in 1970 that graphite, in a lithium metal half cell using propylene carbonate (PC), reduced the electrolyte during discharge at a rate which linearly increased with the current. They proposed that the following reaction was taking place:

C 4 H 6 O 3 + 2 e − ⟶ CH 3 − CH = CH 2 + CO 3 2 − {\displaystyle {\ce {C4H6O3 + 2e- -> CH3-CH=CH2 + CO3^{2-}}}}

The same reaction was later proposed by Fong et al in 1990, where they theorized that the carbonate ion was reacting with the lithium to form lithium carbonate, which was then forming a passivating layer on the surface of the graphite. PC is not commonly used in batteries today as the molecules can intercalate into the graphite layers and react with the lithium there to form propylene and acts to delaminate the graphite.

The insulating properties of the SEI allow the battery to reach more extreme voltage gaps without simply reducing the electrolyte. This ability of the SEI to improve the voltage window of batteries was discovered almost by accident but plays a vital role in high voltage batteries today.

Solid electrolytes

Recent advances in battery technology involve using a solid as the electrolyte material. The most promising of these are ceramics. Solid ceramic electrolytes are mostly lithium metal oxides, which allow lithium-ion transport through the solid more readily due to the intrinsic lithium. The main benefit of solid electrolytes is that there is no risk of leaks, which is a serious safety issue for batteries with liquid electrolytes. Solid ceramic electrolytes can be further broken down into two main categories: ceramic and glassy. Ceramic solid electrolytes are highly ordered compounds with crystal structures that usually have ion transport channels. Common ceramic electrolytes are lithium super ion conductors (LISICON) and perovskites. Glassy solid electrolytes are amorphous atomic structures made up of similar elements to ceramic solid electrolytes but have higher conductivities overall due to higher conductivity at grain boundaries. Both glassy and ceramic electrolytes can be made more ionically conductive by substituting sulfur for oxygen. The larger radius of sulfur and its higher ability to be polarized allow higher conductivity of lithium. This contributes to conductivities of solid electrolytes are nearing parity with their liquid counterparts, with most on the order of 0.1 mS/cm and the best at 10 mS/cm. An efficient and economic way to tune targeted electrolytes properties is by adding a third component in small concentrations, known as an additive. By adding the additive in small amounts, the bulk properties of the electrolyte system will not be affected whilst the targeted property can be significantly improved. The numerous additives that have been tested can be divided into the following three distinct categories: (1) those used for SEI chemistry modifications; (2) those used for enhancing the ion conduction properties; (3) those used for improving the safety of the cell (e.g. prevent overcharging).^{[citation needed]}

Electrolyte alternatives have also played a significant role, for example the lithium polymer battery. Polymer electrolytes are promising for minimizing the dendrite formation of lithium. Polymers are supposed to prevent short circuits and maintain conductivity.

The ions in the electrolyte diffuse because there are small changes in the electrolyte concentration. Linear diffusion is only considered here. The change in concentration c, as a function of time t and distance x, is

∂ c ∂ t = D ε ∂ 2 c ∂ x 2 . {\displaystyle {\frac {\partial c}{\partial t}}={\frac {D}{\varepsilon }}{\frac {\partial ^{2}c}{\partial x^{2}}}.} !{\displaystyle {\frac {\partial c}{\partial t}}={\frac {D}{\varepsilon }}{\frac {\partial ^{2}c}{\partial x^{2}}}.}

In this equation, D is the diffusion coefficient for the lithium ion. It has a value of 7.5×10⁻¹⁰ m²/s in the LiPF
₆ electrolyte. The value for ε, the porosity of the electrolyte, is 0.724.

Dry-processed electrode manufacturing

Dry electrode manufacturing is a solvent-free electrode preparation process that serves as an alternative to the traditional slurry coating method for lithium-ion batteries. Unlike conventional methods requiring liquid solvents such as N-methylpyrrolidone (NMP) to mix active materials, dry electrode technology relies on mechanical mixing, dry coating, and compaction to form a dense electrode structure.

Process

The typical preparation of dry electrodes involves three steps:

Dry mixing: Active materials, conductive agents, and binders are uniformly blended under solvent-free conditions.

Dry coating: The powder mixture is evenly coated onto the current collector surface under shear force.

Compression/Calendering: The coated layer is compressed to achieve the target thickness and sufficient mechanical strength.

Advantages

The dry electrode process eliminates the need for drying equipment, NMP recovery systems, or procedures for handling thick slurries since it operates entirely without solvents. Its advantages include: significantly reduced energy consumption and manufacturing costs, elimination of the toxic solvent NMP, enhanced environmental sustainability, and the ability to produce thicker electrodes with higher loading capacities.

PTFE fiberization binder

Dry electrodes typically utilize polytetrafluoroethylene (PTFE) as a binder. Under shear stress, PTFE forms a network of elongated fibers that permeates the entire electrode structure. This PTFE fiber network provides the electrode with exceptional mechanical strength, flexibility, and particle adhesion, thereby compensating for structural deficiencies in the absence of slurry mixing.

Biomass additives

Recent studies have introduced biomass additives such as starch, cellulose, and flour to enhance the pore structure and flexibility of electrodes. These additives promote intermolecular crosslinking, reduce tortuosity, and improve electrolyte wettability. For instance, incorporating 1 wt.% flour into PTFE dry electrodes significantly increases mechanical strength, accelerates lithium-ion transport, and enhances high-rate performance.

Performance improvements

Leveraging the synergistic interaction between PTFE fiber networks and biomass additives, dry electrodes demonstrate: reduced bending, accelerated lithium-ion transport, enhanced rate performance in high-voltage cathodes (e.g., NCM811), superior cycling stability due to diminished particle cracking, and more uniform electrolyte permeation with improved interfacial stability. These findings indicate that dry electrode technology holds significant potential for scalable, sustainable battery manufacturing.

Challenges and Future Directions

Although dry electrode manufacturing excels in environmental friendliness and high energy density, it still faces challenges in industrialization and mass production. First, high-thickness electrodes may exhibit localized density variations during pressing, leading to reduced cycle life or unstable electrochemical performance. Second, PTFE binders carry higher costs, and while biomass additives help improve pore structure, their ratios require optimization to balance performance and processing stability. Furthermore, scaling up laboratory-scale preparation methods to industrial production necessitates addressing issues such as coating uniformity, consistent pressing, and quality control.

Future development directions include: researching low-cost or biodegradable binder alternatives; developing thick electrode designs that balance high energy density with mechanical stability; introducing automated quality monitoring technologies to support mass production; and utilizing advanced characterization methods to optimize pore structure and ion transport properties. These improvements are expected to drive the widespread adoption of dry-process electrode technology in commercial lithium-ion batteries.

Battery designs and formats

Lithium-ion batteries can be small with just one cell or large by connecting many cells together. Bigger batteries link cells together in groups to make modules, and then link modules together to form packs. These packs can also be linked to raise the voltage.

Batteries often include extra parts like temperature sensors, heating or cooling systems, and monitors to keep them safe from problems like getting too hot or short circuits.

Li-ion cells come in different shapes and sizes. Small coin cells are tough but have less energy and are used in watches and calculators. Cylindrical cells are common in e-bikes, electric vehicles, and older laptops. Larger cylindrical cells have big terminals. Flat or pouch cells are used in cell phones and newer laptops. Some cells are in rigid plastic cases with large terminals, often used in electric vehicle packs.

Cylindrical cells are made in a "swiss roll" style, rolling layers together into a spool. Pouch cells have the most energy but need a case to stay stable. Electric vehicles today use cylindrical, prismatic pouch, or prismatic can cells. The smallest Li-ion cell is very tiny, made by Panasonic.

Uses

Lithium-ion batteries are used in many things, such as consumer electronics, toys, power tools, and electric vehicles.

They are also sometimes used for backup power in telecommunications and might be used for storing energy for the power grid in the future. Some submarines have lithium-ion batteries too.

Performance

Lithium-ion batteries come in many types because they can use different materials for their electrodes. This means their energy and voltage can change a lot.

These batteries have a higher voltage than other types, like lead-acid or nickel-metal hydride batteries. But as they are used and get older, their internal resistance goes up. This makes the battery give less power and can even stop working properly.

Some lithium-ion batteries, like those with lithium iron phosphate and graphite, have a voltage of about 3.2 V. Others, like those with NMC oxide and graphite, have about 3.7 V. It used to take hours to charge these batteries, but now some can be fully charged in just 45 minutes or even faster. Researchers have made small batteries that can get a lot of charge very quickly.

Over the years, lithium-ion batteries have gotten much better. From 1991 to 2018, they became much cheaper and could store more energy. This improvement helps make electric vehicles more affordable.

Different sizes and shapes of lithium-ion batteries can have different energy levels. For example, jelly roll cells often have more energy than other shapes because they are packed more tightly.

Lifespan

Lithium-ion batteries have a long life, but they slowly lose their ability to hold a charge over time. This is called battery lifespan and is usually measured by how many times you can fully charge and discharge the battery before it drops to 80% of its original capacity.

Batteries can also lose capacity just by sitting around, especially if they are kept in a charged state or in very hot or cold places. Many things affect how long a battery lasts, such as temperature, how fast you charge or discharge it, and how much of the battery’s charge you use each time. Scientists study these batteries to understand why they degrade and how to make them last longer.

Safety

Lithium-ion batteries can be safe, but there are some risks to know about. One main risk happens when the battery is charged too fast or in very cold temperatures. This can cause tiny pieces of lithium to build up inside the battery. These pieces, called dendrites, can poke holes in the battery and cause big problems like overheating or even fires.

Another risk is if the battery gets damaged or crushed. This can also lead to overheating and fires. That’s why it’s important to follow the instructions for charging and storing these batteries. Some larger batteries have special safety parts to help prevent these problems, like devices that stop the battery from working if it gets too hot or pressurized.

Fire hazard

Lithium-ion batteries can catch fire if they are not treated properly. They contain a special liquid that can catch fire if the battery is damaged or charged incorrectly. This can happen if the battery gets too hot, is charged too fast, or is damaged in an accident. Because of these risks, there are strict rules for testing and shipping these batteries.

In the past, some companies had to recall their batteries because they caused fires. For example, in 2016, a popular phone had to be recalled because its batteries were catching fire. Researchers are working on new types of batteries that are less likely to catch fire.

Damaging and overloading

If a lithium-ion battery is damaged, crushed, or used too much without proper safety features, it can cause big problems. Throwing these batteries in the regular trash can also be dangerous because recycling centers might damage them, leading to fires. To stay safe, always follow the instructions for disposing of these batteries.

If a battery gets too hot or is charged too much, it can break apart and even catch fire. That’s why many batteries have special parts that turn them off if they get too hot or charged too much. But if these safety parts are not working right, the battery can still be dangerous.

Voltage limits

Lithium-ion batteries work best when their voltage is just right. If the voltage is too high or too low, the battery can break down and become unsafe. Storing these batteries for a long time can also drain them too low, making them hard to charge again. It’s important to keep these batteries within their safe voltage range to avoid problems.

Recalls

In the past, some companies had to recall millions of lithium-ion batteries because they were unsafe. These batteries could get contaminated during making, which sometimes caused them to catch fire. It’s always best to follow the manufacturer’s instructions and safety guidelines when using these batteries.

Supply chain

Most of the materials needed to make lithium-ion batteries come from just a few places. About half comes from Australia, with more coming from Chile and China.

Getting these materials can be hard on the environment. Mining can hurt water and air quality, damage landscapes, and use a lot of water. Some places where mining happens don’t have enough water already. Also, mining can create waste that is hard to clean up.

There are ways to recycle old batteries to help reduce the need for new materials. Recycling can recover valuable metals and other parts, making it better for the planet. However, recycling isn’t always easy and needs special processes to be done safely.

Research

Main article: Research in lithium-ion batteries

Scientists are always finding new ways to make lithium-ion batteries better. They want to make these batteries safer, longer-lasting, cheaper, and faster to charge. One exciting idea is to use solid-state batteries, which could be a big step forward. Many companies are working hard to make these new batteries popular.

Researchers also look at ways to make batteries last longer and hold more energy. They study special liquids and materials that are safer to use. By mixing different materials, scientists can create batteries that work better and last longer. They even look at how the oxygen in these batteries can help make them even stronger. All of this research helps make batteries safer and more powerful for things like phones and cars.