Cathode Materials for Li-Ion Batteries

Table 1. Electrochemical characteristics of the three classes of insertion compounds.

Framework	Compound	Specific capacity a (mAh g−1)	Average potential (V vs. Li0/Li+)
Layered	LiCoO2	272 (140)	4.2
	LiNi1/3Mn1/3Co1/3O2	272 (200)	4.0
Spinel	LiMn2O4	148 (120)	4.1
	LiMn3/2Ni1/2O4	148 (120)	4.7
Olivine	LiFePO4	170 (160)	3.45
	LiFe1/2Mn1/2PO4	170 (160)	3.4/4.1

a: Value in parenthesis indicates the practical specific capacity of electrode. 
I.The Cell Potential (Goodenough Diagram) 

−eV_oc = μ_Li(C) −μ_Li(A) = ∆μ_e + ∆μ_Li⁺

the chemical potential of the exchanged Li-atoms in anode (A) and cathode (C) is conceptually divided to the involved occupation of sites by Li⁺-ions and the valence electronic density of states (DOS) by electrons

Schematic diagram showing the electronic density of states and Fermi energies for an oxide-based electrode (Li_xNi_0.5−yMn_1.5−yCr_2yO₄ spinel material). The Li permeable solid electrolyte interface (SEI) layer formed on the electrode surface preserves the overall reversible reaction.

 - Successive redox couples are separated by an on-site effective Coulomb correlation energy U that can be large when augmented by either a crystal-field splitting or an intra-atomic exchange splitting. However,  when the Fermi energy E_FC of the cathode material approaches the top of the anion p bands of the host, the p-d covalent mixing may transform the correlated d electrons at E_FC into band electrons occupying one-electron states

 - In the absence of a crystal-field splitting of the d orbitals at E_FC, which is the case for Ni(IV) to Ni(II), the one-electron states are not separated by any on-site energy U and there is no step in the voltage of the battery. E_FC is moved from one formal valence state to another upon the reduction or oxidation of the host.
II.Crystal Structure and Electronic Properties

Crystal structure of the three lithium-insertion compounds in which the Li⁺ ions are mobile through the 2-D (layered), 3-D (spinel) and 1-D (olivine) frameworks

1.Layered Compounds

Li[M]O₂ (M = Co, Ni) oxides are isostructural to the layered α-NaFeO₂ (space group R3m, No. 166) with the oxygen ions close-packed in a cubic arrangement and the TM and Li ions occupying the octahedral sites of alternating layers with an ABCABC… stacking sequence called “O3-type” structure

A.Li_xCoO₂ (LCO)

• In LiCoO₂, the cobalt is trivalent in the electronic configuration (t_2g)⁶(e_g)⁰, i.e., in the  low-spin state (S = 0)
• However, LCO adopts the rhombohedral symmetry in the high temperature form, with Li in 3a, Ni in 3b and O in 6c sites 
• Note that the low-temperature form (LT-LCO) adopts the spinel lattice with the cubic symmetry (S.G. Fd3m) 

A lithium ordering and stacking sequences leading to an equivalent environment for all Co ions is preferred in order to achieve a maximum of charge delocalisation and to minimize the energy

 - In Li_xCoO₂, no coupling between Co:e_g and Li:2s states occurs and the lowest-energy is reached in the interplanar stacking that leads to as many equivalent Co sites as possible [In Li_0.5CoO₂, Co tends to have an intermediate oxidation state of +3.5 that induces a transition to a monoclinic structure]
 - LiCoO₂ suffers from the dissolution of the metal ion in the electrolyte that induces oxygen release, which becomes more important upon increasing the temperature. Thus, surface modification by metal-oxide coating such as ZrO₂, Al₂O₃, TiO₂, etc. was demonstrated being an effective strategy to avoid the cathode  breakdown
B.Li_1−zNi_1+zO₂ (LNO)
LiNiO₂ (LNO) is isostructural with LiCoO₂ and has the O3-type-oxygen packing (advantage: high lithium chemical potential provides a high cell voltage)
 - LNO suffers from a few drawbacks:

•  (i) difficulty to synthesize LiNiO₂ with all the nickel ions in the Ni³⁺ valence state and distributed in a perfectly ordered phase without a mixing of Li⁺ and Ni³⁺ ions in the lithium plane; 
•  (ii) Jahn-Teller distortion (tetragonal structural distortion) associated with the low spin Ni³⁺:d⁷ (t_2g⁶e_g¹) ion.
•  (iii) Irreversible phase transitions occurring during the charge-discharge process.
•  (iv) Exothermic release of oxygen at elevated temperatures and safety concerns in the charged state 

Solution: mixed LiNi_1−yCo_yO₂ phases allow overcoming the main drawbacks exhibited by both LiCoO₂ and LiNiO₂ oxides
 2.LiMn₂O₄ (LMO) 
LiMn₂O₄ belongs to the A[B₂]O₄ spinel-type structure and crystallizes in the Fd3m space group (O_h⁷ factor group) with the cubic lattice parameter a = 8.239 Å 

The cubic spinel LiMn₂O₄ structure is described with the Mn and Li cations on the 16d and 8a sites, respectively, and the oxygen ions located on the 32e sites form a nearly ideal cubic close-packed (ccp) sublattice. Lithium ions occupy tetrahedral sites, which share common faces with four neighboring empty octahedral sites at the 16c position

This lattice offers a three-dimensional network of transport paths 16c-8a-16c through which lithium ions diffuse during insertion/deinsertion reactions 
3.LiNi_0.5Mn_1.5O₄ (LNM)
Substitution of 25% Ni for Mn in LiMn₂O₄ spinel has been chosen because this composition implies that Mn is in the 4+ valence, thus avoiding the Jahn-Teller (JT) distortion associated to Mn³⁺.
=>the electrochemical activity is only due to the oxidation/reduction of Ni²⁺ ions leading transfer of 2e⁻ per Ni ion

• the face-centred spinel (S.G. Fd3m) named as “disordered spinel”
• the simple cubic phase (S.G. P₄332) named as “ordered spinel” 

The cation distribution in the P₄332 symmetry is then Li on 8c, Ni on 4b, Mn on 12d, and O(1) and O(2) oxygen ions occupy the 24e and 8c Wyckoff positions, respectively. The net result is thus a significant optimisation of space occupation leading to a reduced unit cell volume. It has been pointed out that phase-pure LNM is difficult to synthesize because impurities such as NiO and/or Li_yNi_1−yO usually exist . The partial replacement of Ni and Mn by Cr in LiNi_0.5−yMn_1.5−yCr_2yO₄ is an effective way to alleviate the problem of oxygen loss generating Mn³⁺ ions in the LNM framework and a voltage plateau at ca. 4 V vs. Li⁰/Li⁺. Thus, it has been demonstrated that the Cr-doping stabilizes the lattice without impacting the capacity significantly, but it decreases the energy density
4. Olivine LiFePO₄ (LFP)

 LFP crystallizes in the orthorhombic system (Pnma space group,  No. 62). It consists of a distorted hexagonal close-packed (hcp) oxygen framework containing Li and Fe located in half the octahedral sites and P ions in one-eighth of the tetrahedral sites. The FeO₆ octahedra, however, are distorted, lowering the regular octahedral O_h to the C_s symmetry

[[[The LiFePO₄ structure consists in three non-equivalent O sites. Most of the atoms of the olivine structure occupy the 4c Wyckoff position except O(3) which lies in the general 8d position and Li⁺ ions occupying only the 4a Wyckoff position (M₁ site on an inversion center). The Fe magnetic ions are in the divalent Fe²⁺ state and occupy only the 4c Wyckoff position (M₂ site in a mirror plane), i.e., the center of the FeO₆ units. As a consequence, Fe is distributed so as to form FeO₆ octahedra isolated from each other in TeOc₂ layers perpendicular to the (001)-hexagonal direction [22]. In addition, the lattice has a strong two-dimensional character, since above a TeOc₂ layer comes another one vertical to the previous one, to build (100) layers of FeO₆ octahedra sharing corners, and mixed layers of LiO₆ octahedra and PO₄ octahedra. The lithium iron phosphate material differs from the primary mineral triphylite Li(Mn,Fe)PO₄ by the fact that triphylite is only rich in iron, with some manganese ions also in the M₂ site [32]. However, while the triphylite is a naturally occurring mineral, LiFePO₄ is an artificial product]]]

Comparison of the energy vs. density of states showing the relative Fermi level of the Co^4+/3+ redox couple for LiCoO₂, the Ni^4+/3+ redox couple for LiNi_0.8Co_0.2O₂, the Mn^4+/3+ redox couple for LiMn₂O₄ and the Ni^3+/2+ redox couple for LiNiPO₄

The stabilization of the higher oxidation state is essential to maximize the cell voltage and the energy density. The location of O:2p energy and a larger raising of the Mⁿ⁺:d energies due to a larger Madelung energy make the higher valent states accessible in oxides. That is why transition-metal oxide hosts were pursued as positive electrode candidates for Li-ion secondary batteries. 

III.Electrochemical Properties and Phase Diagram

1.Lithium Cobaltate (LCO)

 

Charge-discharge curves Li_xCoO₂ at C/24 rate in the range 3.6–4.85 V vs. Li⁰/Li⁺. The sequence of the several phases is indicated as x varies from 1.0–0.05. 

Parameter: Charge-discharge curves Li_xCoO₂ at C/24 rate in the range 3.6–4.85 V vs. Li⁰/Li⁺

=> x varies from 1.0–0.05 

 - LiCoO₂ has shown degradation and fatigue during electrochemical cycling.

 - the valence band (VB) is mainly unchanged with a slight shift of the top of the VB to lower binding energies, which implies a shift of E_FC that provokes the removal of d-electrons due to the change of the oxidation state from Co³⁺ to Co⁴⁺.

• For 1.0 ≥ x ≥ 0.5, the DOS nearly does not change and the charge compensation with Li extraction leads to a removal of electrons from the Co:3d t_2g derived states with the Fermi level moving downwards
• For x < 0.5, a clear increase in hybridisation occurs between the Co:3d and O:2p states associated with a reduction of the  (CoO₆)-slab distances evidenced by the reduction of the c-axis lattice parameter 

=> the charge compensation of the delithiation leads to a removal of electrons from Co-O:d-p hybrid states, which translates to a partial oxidation of the O²⁻ ions

 2. Lithium Manganese Spinel (LMO) 

The spinel LiMn₂O₄ has a strong edge-shared [Mn₂]O₄ octahedral lattice and exhibits good structural stability during the charge-discharge process. LiMn₂O₄ spinels have shown a lack of robustness in their cycle life and irreversible loss of capacity that becomes rapid at elevated temperatures 

 - Li⁺ ions are extracted from the tetrahedral sites of the Li_xMn₂O₄ spinel structure at approximately 4 V in a two-stage process, separated by only 150 mV  at a composition Li_0.5Mn₂O₄

Voltage profile of a Li//LiMn₂O₄ cell discharged at C/24 rate with LMO material synthesized at 700 °C (left).Variation of the lattice parameters as a function of the Li content x during the charge/discharge in LMO cathode (right). 

A.Charge

Lithium insertion into LMO occurs at approximately 3 V. During this process, Li⁺ ions are inserted into the octahedral 16c sites of the spinel structure. Since the 16c octahedra share faces with the 8a tetrahedra, electrostatic interactions between the Li⁺ ions on these two sets of sites cause an immediate displacement of the tetrahedral-site Li⁺ ions into neighboring vacant 16c octahedral sites. The reaction results in a first-order transition to Li₂Mn₂O₄ with a stoichiometric rock-salt composition on the surface of the electrode particle.

B.Discharge

a reaction front of Li₂Mn₂O₄ moves progressively from the surface of the LiMn₂O₄ particle into the bulk. At 3 V, the Li insertion is accompanied by a severe Jahn-Teller distortion as a result of an increased concentration of Mn³⁺:d⁴ ions in the Mn₂O₄ spine1 lattice, which reduces the crystal symmetry from cubic (c/a = 1.0) to tetragonal symmetry (c/a = 1.16) that results in a 16% increase in the c/a ratio detrimental to the electrochemical cycling

. The decrease of the Li ion diffusion coefficient is involved as another contribution to the capacity fade, which is caused by the passive film formation on the active material surface

 - Several reasons have been proposed for the capacity loss of Li//Li_xMn₂O₄ cells in the 4-V  region:

• The major drawback is the disproportionation of Mn³⁺ at the particle surface in the presence of trace amounts of protons (acid attack) into Mn²⁺ and Mn⁴⁺   [  2Mn3+(solid) → Mn4+(solid) + Mn2+(solution)    ] resulting in a leaching out of Mn²⁺ ions from the positive electrode framework into the electrolyte 
• The instability of the delithiated spinel structure by oxygen loss in organic electrolyte solvents in the end of the charge.  
•  The onset of a Jahn-Teller effect at the end of discharge (particularly at high current density). Under dynamic, non-equilibrium conditions above 3 V, it has been proposed that some crystallites can be more lithiated than others, thereby driving the composition of the electrode surface into a Mn³⁺-rich Li_1+xMn₂O₄ region

Solution: 

• The appropriate method to reduce capacity fade of LMO is surface coating of the particles to prevent the Mn²⁺ dissolution by a thin layer of inorganic material such as Al₂O₃, zirconia, MgO, Li₂O-B₂O₃ glass, AlF₃ [improve the elevated temperature storage properties of LMN spinels] 
• choose chemical composition such that Mn remains inactive in the 4+ valence state; this is the case for LiNi_1/3Mn_1/3Co_1/3O₂ and LiNi_1/2Mn_3/2O₄

 - Additional Li could be inserted into the empty octahedral holes of the spinel framework at a potential  of ~3 V vs. Li⁰/Li⁺ accompanied by a structural change from cubic to tetragonal symmetry due to the Jahn-Teller distortion associated with the high-spin Mn³⁺ (t_2g³e_g¹) ions inducing a huge volume change and severe capacity fade

LiMn₂O₄ is a small-polaron semiconductor, electronic conduction occurring via hopping of electrons between e_g orbitals on adjacent Mn³⁺/Mn⁴⁺ cations. Thus, the gradual removal of Li⁺ ions from the structure during deintercalation should result in a decrease in the number of mobile electrons throughout the whole solid 

3.Lithium Mn-Ni-Co Oxides

  - The layered LiNi_yMn_yCo_1−2yO₂ (NMC) compounds with a hexagonal single-phase α-NaFeO₂-type structure have received great attention as 4V-electrode materials to replace LiCoO₂ in Li-ion batteries, owing to its better stability during cycling even at elevated temperature, higher reversible capacity and milder thermal stability at charged state 

 - The main problem that still needs to be solved for such applications of NMC is the cation mixing between nickel and lithium ions, since the ionic radius of Ni²⁺ (0.69 Å) is close to that of Li⁺ (0.76 Å)