Lithium-ion battery components

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.^[13]

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.^[13] However, Na⁺ and Fe³⁺ have sufficiently different sizes that NaFeO₂ can be used in sodium-ion batteries.^[21]

Similarly, LiCrO₂ shows reversible lithium (de)intercalation around 3.2 V with 170–270 mAh/g.^[22] However, its cycle life is short, because of disproportionation of Cr⁴⁺ followed by translocation of Cr⁶⁺ into tetrahedral sites.^[23] On the other hand, NaCrO₂ shows a much better cycling stability.^[24] LiTiO₂ shows Li+ (de)intercalation at a voltage of ~1.5 V, which is too low for a cathode material.

These problems leave LiCoO
₂ and LiNiO
₂ 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.^[25] For this reason, the current trend among lithium-ion battery manufacturers is to switch to cathodes with higher Ni content and lower Co content.^[26]

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),^[27][13] cobalt- between +2 and +3, while Mn (usually >20%) and Al (typically, only 5% is needed)^[28] 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.^[29] NCM and NCA batteries are collectively called ternary lithium batteries.^[30][31]

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:M<0.5. Under such conditions a new semi-reversible redox transition at a higher voltage with ca. 0.4-0.8 electrons/metal site charge appears. This transition involves non-binding electron orbitals centered mostly on O atoms. Despite significant initial interest, this phenomenon did not result in marketable products because of the fast structural degradation (O2 evolution and lattice rearrangements) of such "lithium-rich" phases.^[32]

Cubic oxides (spinels)

LiMn₂O₄ adopts a cubic lattice, which allows for three-dimensional lithium-ion diffusion.^[33] 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 <130 mA h g–1. However, Mn³⁺ is not a stable oxidation state, as it tends to disproportionate into insoluble Mn⁴⁺ and soluble Mn²⁺.^[25][34] 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.^[13] 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.^[29] 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.^[35]

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.^[36] 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.^[37] 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,^[20] it has begun to be widely used in electric vehicles since the 2020s.^[19]

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).^[64] 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.^[65] They proposed that the following reaction was taking place:

${\displaystyle {\ce {C4H6O3 + 2e- -> CH3-CH=CH2 + CO3^{2-}}}}$

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.^[66] 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^[67].

The insulating properties of the SEI allow the battery to reach more extreme voltage gaps without simply reducing the electrolyte.^[68] 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.

The structure of the SEI is commonly described by two models developed before direct imaging techniques became available. In the mosaic model proposed by Peled, the interphase is a heterogeneous mixture in which crystalline inorganic decomposition products such as lithium oxide (Li₂O) and lithium carbonate (Li₂CO₃) are dispersed within an organic matrix.^[69] In the alternative multilayer (or layered) model proposed by Aurbach, the decomposition products are instead arranged in distinct layers.^[70] More recent reviews often reconcile these pictures with a bilayer description, in which a dense inorganic inner layer conducts lithium ions while blocking electrons, and a porous organic outer layer accommodates the volume changes that occur during cycling.^[71]

Solid electrolytes

Recent advances in battery technology involve using a solid as the electrolyte material. The most promising of these are ceramics.^[72] 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.^[73] 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.^[74] 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.^[75] 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.^[76] An efficient and economic way to tune targeted electrolytes properties is by adding a third component in small concentrations, known as an additive.^[77] 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.^[53]

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

${\displaystyle {\frac {\partial c}{\partial t}}={\frac {D}{\varepsilon }}{\frac {\partial ^{2}c}{\partial x^{2}}}.}$

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.^[78]