In order to meet the increasing mileage and cost requirements, nickel-based cathode materials (ie LiNixCoyMnzO2, x + y + z = 1 and LiNixCoyAlzO2, x + y + z = 1) have gradually emerged. In order to achieve a higher energy density (ie 350 Wh kg-1 or 750 Wh L-1), the current long-range electric vehicles use high nickel content (Ni>80%) cathode materials to achieve the same level as that of internal combustion engine vehicles.
Electrochemical and kinetic properties of nickel-rich materials
Many studies have confirmed that while higher nickel content brings greater specific capacity, the stability of electrode materials will also decrease. According to the in situ XRD, it can be seen that the lattice size variation of the c-axis increases with the increase of Ni content. Further studies determined that the c-axis lattice size shrinkage is mainly derived from the phase transition from H2 to H3 phase, and the onset voltage for this shrinkage will decrease with the increase of nickel content.
For example, for NCM622 material, the phase transition from H2 to H3 phase is expected to appear at 4.6 V, while for NCM811 material, the phase transition voltage from H2 to H3 appears at about 4.2 V. Phase transitions cause changes in the material volume, which in turn affects the overall electrochemical and mechanical stability of the material.
In layered nickel-based oxide cathodes, lithium ions can be transported through bulk phases and grain boundaries. In the bulk phase transport, lithium ions mainly have two migration methods, from one octahedral site to another octahedral site through an intermediate-tetrahedral site or an oxygen dumbbell. In the initial stage of charging, lithium ions prefer to use dumbbell-shaped oxygen sites to perform octahedral-octahedral transitions, and when one-third of the lithium ions are extracted, lithium ions transition through the intermediate tetrahedral positions.
The activation energy of lithium ion transport has a great relationship with the size of Li-O octahedron and Li-O tetrahedron, and the large interlayer size will have relatively low activation energy. As for lithium ions transported at grain boundaries, they usually have a better transport environment than lithium ions in the bulk phase, and only need to overcome a relatively low transport barrier. In addition to the effect of location, the transport of lithium ions is also related to SoC.
During the charging process, with the increase of delithiation amount, the diffusion coefficient of lithium ions first increases and then decreases rapidly above 4.2 V, which is related to the lattice size in the c-axis direction. changes are consistent. During discharge, the Li-ion diffusion coefficient is initially high due to the high Li-ion concentration gradient at the cathode-electrolyte interface, and decreases towards the end of discharge due to the high Li-ion concentration gradient at the cathode-electrolyte interface. This is caused by the redistribution of lithium ions in the bulk phase.
Inhomogeneity at multiple scales
At the electrode scale, scientists used advanced characterization and statistical methods to systematically study the macroscopic behavior of hundreds of cathode particles under fast charging conditions. These studies point out that under fast charging conditions, there will be particles with different degrees of damage in the electrode at the same time. In the direction perpendicular to the electrode, the "severely damaged" particles are more distributed in the direction close to the separator, while the "least damaged" particles are more distributed in the direction close to the current collector.
In addition, as the number of cycles increases, the number of severely damaged particles will increase significantly, and the distance between these severely damaged particles will decrease. Due to the mutual compensation between the electrochemical activity of particles and mechanical damage, particles with different damage degrees will gradually tend to similar damage degrees with the cycle. If this phenomenon is explained from the degree of lithiation, the difference in the degree of lithiation between particles will be amplified under fast charging conditions.
It has been proposed that this is due to the phenomenon of electro-autocatalysis, where some particles that are already at a higher degree of lithiation will continue to be charged due to the promoted kinetic conditions, resulting in these particles at a higher delithiation rate state, and an irreversible phase transition occurs.
Oxygen loss caused by thermodynamic instability
Singlet oxygen (singlet oxygen, 1O2), peroxide anodic radicals (O-O-) and peroxide bonds (MO-OM) between transition metals are experimentally verified that several kinds of lattice oxygen exist form. Depletion usually leads to problems such as phase transition, gas production and loss of transition metals.
- Phase transition
The phase transition is from layered phase to spinel phase to rock-salt phase. Usually in the highly delithiated state, the layered phase will first change to the spinel phase, resulting in Ni/Li mixing.
Without exception, the OER (oxygen evolution reaction) reaction can always be observed at the onset of the phase transition from H2 to H3. The irreversible phase transition is aggravated during the battery fast charging process, resulting in a thicker phase transition layer, and the oxygen loss behavior is also observed at deeper depths.
- Gas production
The main gases produced are O2, CO2 and CO. Carbon oxides are produced due to the decomposition of the electrolyte, which is due to the local high-valence nickel caused by local overcharging, thereby oxidizing and decomposing the electrolyte. Oxygen is due to the loss of lattice oxygen. Gas production will also be amplified due to the effect of fast charging.
- Transition metal dissolution
This phenomenon originates from HF corrosion. H2O and LiPF6 produce HF to corrode the positive electrode, dissolve the transition metal, and the transition metal migrates to the negative electrode and is reduced to metal, resulting in repeated charge and discharge of SEI, consuming lithium ions and electrolyte molecules.
- Synthesis of single crystal particles
By controlling the crystal plane orientation of the synthesized single crystal, the volume expansion during charging and discharging can be controlled, and its stability can be increased, so as to ensure that the particles can have better integrity even under the volume change caused by fast charging. Synthesize single-crystal particles with more excellent kinetically oriented crystal planes, thereby increasing their ion transport rate and promoting the rate performance.
- Modification of particle morphology
By constructing special transport channels, or constructing nano-scale particles, the ion transport rate of the positive electrode material can be increased and the rate performance can be increased.
- Cation doping
Improve thermodynamic stability, that is, maintain structural integrity under the harsh conditions of fast charging. Or change the structural orientation of primary particles by doping to make it more conducive to the transport of lithium ions.
Improve the surface stability, and at the same time, the coating layer should preferably cover the primary and secondary particles, so as to fully inhibit the localized high-valence nickel oxidation decomposition electrolyte and oxygen loss caused by fast charging. At the same time, some ion-conducting coatings can facilitate ion transport and improve kinetic performance.
- Electrolyte additives
By using electrolyte additives, a protective layer is formed on the surface of the cathode particles during the first electrochemical process, thereby improving electrochemical, chemical and structural stability. At the same time, the protective layer formed by some additives can promote the diffusion of lithium ions from the cathode to the electrolyte to improve its dynamic performance.