Deficiency of high nickel NCM ternary materials and aluminum ion doping modification

In order to meet the demand of high energy density of lithium ion batteries, the proportion of active element nickel in NCM ternary materials has been increasing in recent years, and NCM9 series with nickel content of 90% or more has become the next generation of flagship products of many battery enterprises.  The high nickel content ternary cathode material has higher initial discharge capacity, but it also brings a series of problems.

Deficiencies of high nickel oxide NCM ternary materials

There is a tradeoff between the specific capacity of NCM ternary cathode materials and the capacity retention rate and thermal stability. With the increase of nickel content, the high nickel layered transition metal oxides provide a higher specific capacity at the expense of the capacity retention rate and thermal stability.  

The main reason is that high nickel content leads to increase in cationic degree mixed materials, and three yuan high nickel anode materials are more likely to happen in the process of electrochemical cycle from H2 phase to H3 of irreversible phase transition, the volume anisotropy degree of change is bigger, causes the degeneration of the material structure, the formation of micro cracks, side effects and a gas is generated and a series of problems,  The cycle life of lithium ion battery is reduced.

Cation mixing  

The NCM ternary cathode material is prone to cation mixing during the synthesis stage and electrochemical cycle, that is, the transition metal ion at 3a position in the layered structure and the lithium ion at 3B position are partially occupied.  

The radius of Ni2+ ion is 0.69A, and the radius of Li+ ion is 0.76A. The two kinds of ions have similar radii, and they tend to occupy each other's sites, resulting in Li+/Ni2+ mixing.  

In the sintering process of high nickel ternary layered materials, Ni2+ in the material is difficult to be completely oxidized to Ni3+, and part of the Existing Ni2+ in the material may migrate from the transition metal layer in the layered structure to Li+ layer to form Li+/Ni2+ mixed arrangement.  During the electrochemical cycle, Li+ escapes from the material layer, resulting in a large number of vacancies. Part of Ni2+ at 3a position will migrate spontaneously to Li layer and occupy Li+ position. Excessive Ni2+ occupying Li layer will intensify the transformation of layered structure to spinel structure or even halite phase structure, resulting in serious capacity attenuation.

Surface side reaction  

In the process of synthesis and electrochemical cycling, NCM ternary cathode material will inevitably contact with the components in the air and electrolyte, resulting in side reactions, which will seriously affect the cycling stability and safety performance of the battery.  In the preparation of high ni N ternary layered materials, in order to obtain ordered layered materials and compensate for the volatilization of lithium source at high temperature, lithium source is usually excessive relative to transition metal in the process of mixed lithium sintering, which will lead to the presence of residual lithium on the surface of the cathode material.  

Therefore, if the high nickel ternary cathode material is exposed to the air, the residual lithium on the surface of the material will react with CO2 and H2O in the air to form LiOH, Li2CO3 and other by-products, which will hinder the migration of Li+. There will also be side reactions and gas generation with electrolyte and polymer binder, leading to the deterioration of battery performance.

In addition, high nickel materials in a large number of Ni4 + at high oxidation state, in the condition of charged on the interface between the electrode and the electrolyte will form a permanent Ni - O surface, formation of Ni - O rock salt no electrochemical activity and ionic conductivity, will enhance the dynamics of the lithium ion diffusion barriers, led to the decrease of the electrochemical properties and battery impedance increase.  

The high activity Ni4+ and Co4+ in the high nickel material can easily catalyze the oxidation decomposition reaction of the electrolyte, resulting in by-products covering the surface of the anode and cathode materials.  At the same time, the released O2 can also react with flammable organic electrolytes and generate large amounts of heat, causing serious safety problems.  

During the electrochemical cycle, the ni-O phase on the surface of the ni-rich material will react with the electrolyte directly.  Compounds produced by side reactions are attached to the surface of the electrode, such as LiF, LiO2, Li2CO3 and LiOH, and their composition is closely related to the composition of the electrolyte used.  These side reaction products are insulating, which leads to the obstruction of diffusion and deterioration of electrochemical performance of Li+.

Micro cracks  

The formation of microcracks is an important cause of battery capacity attenuation, which is caused by strain stress and pressure at grain boundary of material during cyclic charging and discharging process.  

In the synthesis process of high nickel ternary cathode material, due to oxygen precipitation and the existence of heterogeneous phase, the huge pressure and thermal stress in the material will promote the formation of microcracks.  At the same time, in the charging and discharging process of the high nickel ternary cathode material, the material lattice expands and contracts, accompanied by volume expansion and contraction, which also promotes the formation and expansion of microcracks.  Microcracks expand along the reaction zone between the active material and electrolyte, and accelerate particle rupture and electrolyte decomposition.  Microcracks will also cause the destruction of the layered structure of the material, and the phase transformation will cause the electrochemical capacity of the material to decay.  Microcracks not only form at the grain boundary of the material, but also extend to the interior of the particles, leading to the rupture and crushing of the particles once. The crushed particles cannot participate in the electrochemical reaction, and ultimately lead to capacity attenuation.  

Thermal stability problem  

The structural stability and thermal stability of NCM ternary cathode materials decrease with the increase of nickel content, but increase with the increase of cobalt and manganese content. The high specific capacity of high nickel ternary cathode materials is at the expense of thermal stability and safety characteristics.  

The poor thermal stability of high-nickel NCM materials is mainly attributed to the degradation of the surface structure during heating. At lower temperatures, the surface of high-nickel materials begins to transform into rock salt structure and release oxygen.  The content of Ni4+ ions in the material plays a dominant role in affecting the thermal stability of the material. During the electrochemical cycle, Ni4+ ions will be reduced to Ni2+, and O2- will be oxidized from the lattice to keep the charge neutral.  With the increase of nickel content in NCM ternary materials, the hybridization degree of Ni 3D and O 2P orbitals is higher, and the covalence of Ni-O bond is increased, which further promotes the release of oxygen.  A large amount of oxygen evolution will produce large amounts of oxygen vacancy, which reduces the activation energy of the transition metal cation migration bases, and speeding up the material from the layered structure to the spinel structure and the phase transition process of rock salt phase structure, the release of oxygen and the structure of the anode material degradation form a vicious circle, affecting the thermal runaway of the battery temperature, resulting in a decline in battery thermal stability.

High nickel oxide powder NCM ternary material modified by aluminum ion doping

Doping other elements in the crystal structure is a common method for material modification. Doping trace elements can stabilize the structure of high nickel NCM materials and improve the reversibility of electrode reaction.  

Among the doped elements, Al element is the most widely studied.  In a sense, NCM doping Al element can be regarded as the marriage of NCM ternary and NCA ternary, doesn't it look like the NCMA of four-element lithium battery promoted by LG New Energy of South Korea?

Advantage 1: reduce cost  

NCMA replaces part of Co element with Al element on the basis of ternary battery, which can reduce the use of rare and toxic Co element and reduce the cost.  

Advantage 2: improve battery stability  

The radius of Al3+ is similar to that of Co3+, showing electrochemical inertia. Al3+ can stably bond with surrounding O2- even at high voltage, which can stabilize the structure and improve the cycle stability of the battery.  In addition, the strength of Al-O bond is stronger than that of Ni (Co, Mn) -O bond, which tends to improve the thermal stability.

Advantage 3: reduce intergranular crack  

After Al³⁺ doping in NCM, the internal morphology of spherical particles changes, and the primary particles are columnar along the radial direction, which is different from the equiaxed shape in the undoped materials.  The change of grain morphology can reduce the occurrence of radial intergranular cracks.