Introduction

Lithium nickel manganese cobalt oxides, or short, Li-NMC,  are electrode materials commonly used in rechargeable batteries. Li-NMC batteries have Li-NMC as cathode material and belong to the category of lithium-ion batteries (LIB). They have a similar setup as other Li-Ion-based batteries. Li-NMC is a complex layered metal oxide consisting of lithium, nickel, manganese, and cobalt and has the general formula of LiNixMnyCozO2.[1] It was developed in the period of 2001-2008 in efforts to reduce the cobalt content in cathodes for LIBs while increasing the capacity of batteries.[2]Li-NMC batteries are one of the most important among LIBs and are part of the next generation. They are mainly used for transportation applications (e.g. electric vehicles).[3][4]They play an important role in a greener future through the reduction of greenhouse gases emitted by combustion cars.

Working Principle of Li-ion batteries

Li-NMC batteries are part of Li-Ion batteries (LIBs). LIBs consist of four main components, the cathode, anode, electrolyte, and separator. The electrodes have a potential difference and are kept from contact through the separator. The electrolyte is needed as a contractor of the Li-ions.^[5]The discharging process is described in Figure 1. During the discharging process, electrons move from the anode (negative electrode), through an external load, e.g. a car. The lithium ions released from the anode during this process move through the electrolyte and the separator to the cathode (positive electrode). ^[6]For recharging the battery the electrodes are connected via an external electrical supply (power socket). The aforementioned process is reversed. Electrons are released from the positive electrode and move externally to the anode, while the Li⁺ ions move towards the negative electrode, forming a Li_xC₆ compound at the electrode.^[5] 

Figure 1: Schematic of the discharging process of a Li-ion battery. Created by Freya Huck.

Electrochemistry

The discharging process follows the following reactions: 

Cathode: Ni_xMn_yCo_zO₂ + Li⁺ + e^- → LiNi_xMn_yCo_zO₂

Anode: LiC₆ →  Li⁺ + e^-

The oxide in the cathode is reduced during the discharging process and in the anode, graphite is oxidized. For the charging process, these reactions are reversed.^[5]The transition metals have different effects on the battery. Nickel determines mainly the capacity, manganese the thermal stability and cobalt the rate capabilities of the material.  Each effect can be tailored individually by the composition of the material. To counteract the effect of thermal destabilization of the material of high Ni content Ni-rich NMCs can be prepared within a Mn-rich phase.^[2]This example shows nicely, how versatile Li-NMC can be. However delamination between the core and the shell are a problem to be solved.^[2]In Li-NMC cobalt has the oxidation state Co³⁺ and is replaced gradually with Ni²⁺ and Mn⁴⁺.The most commonly used stoichiometry of  LiNi_xMn_yCo_zO₂  (x,y,z=1/3) exhibits a capacity of 150 mAh/g when cycled in the voltage  range of 3.0 V – 4.3 V_.^[2]

Electrolyte 

The electrolyte in a battery is the media through which the lithium ions move back and forth between the oppositely charged electrodes. Electrolytes play a major role in the safety of LIBs.^[6]High ionic conductivity (10⁻³ S cm⁻¹), low electronic conductivity (10⁻¹⁰ S cm⁻¹), thermal stability, cheap production, and environmental friendliness are the main requirements for a good electrolyte.^[6] In most commercially used batteries lithium salts (LiPF₆) are dissolved in a non aqueous organic solvent like for example dimethyl carbonate (DMC) or ethylene carbonate (EC)^[6]. Carbonate-based electrolytes have the advantage, that they dissolve Li-salts and they can also form protective layers on graphitic anode materials.^[6]

Anode

There are three main categories for anode types which are, insertion like graphite, conversion , and lithium alloys like  Li_xM_y (M = Al, Ga, In, Si, Ge, Sn, Pb, Sb, and Bi)^[6]. One of the most commonly used anode material is graphite. Li-ions can be intercalated in the layers, which each are approx. 0.34nm apart, forming Li_xC₆. The maximum theoretical capacity of the graphitic anode is 372 mAh g⁻¹.^[6]However, during the first discharge process, a protective layer consisting of inorganic and organic compounds and components respectively is formed. This reduces the capacity but prevents the decomposition of electrolyte and delamination of the anode.^[6] The structure of graphite can be found here: Allotropy of carbon.

Cathode

In Li-NMC batteries, the working potential is determined greatly by the cathode material used.^[6]Here, Li-NMC as the cathode material will be discussed. It is important to note that different stoichiometry of the metals leads to different properties of the battery. Increasing the nickel content for example reduces the cost of the battery, however a high nickel content (0.8)  leads to poor thermal properties, which could lead to fire hazards.^[6] 

Synthesis of Li-NMC

The goal of the synthesis of Li-NMC is to create a high-energy density cathode material. The method affects the physical and electrochemical properties of the cathode material, and therefore the quality of the battery directly.^[7]The size and shape of primary and secondary particles are also affected. Primary particles are the building block of the bigger secondary particles and are composed of layers of transition metals and lithium. Secondary particles are bigger than primary particles which are normally nano sized.The most commonly used method is coprecipitation combined with sintering based on current publications. Sol-gel, hydrothermal, and spray pyrolysis methods are also used for the synthesis of this cathode material, but less common.^[7]

Coprecipitation of Li-NMC

Is a fairly new method, compared to the widely known solid state and sol-gel synthesis. Coprecipitation methods can be divided in three different types: hydroxide (OH-coprecipitation), carbonate, and oxalate coprecipitation. The main difference between these is what precursor is generated, what chelating agent is used and what conditions are necessary.

Synthesis of cathode material can be described in two main steps:^[7]

1. Generating precursors through coprecipitation of transition metals
   1. Mixing cobalt salt, nickel salt, and manganese salt in desired stoichiometry.
   2. Formation of ionic solution.
   3. Adjustment of pH leads to coprecipitation of uniform particles with aid of chelating agent.
   4. Removal of impurities through washing with deionized water or other solvents and filtering the mixture.
   5. Drying of precursor.
2. Sintering
   1. Mixing and grinding of precursor with lithium hydroxide or lithium carbonate.
   2. Sintering at 600 °C - 1000 °C.

Nitrates^[8], sulfates^[1] or carbonates^[7] of the transition metals can be used for the coprecipitation.

The main advantages using this method are that low sintering temperatures are needed and the holding time is relatively low.The low temperatures are possible because of the homogeneous mixing of the metals.This is good for preserving the morphology of the precursor particles. However the morphology is affected by the sintering conditions such as the atmosphere, time and how many steps are needed.^[7]Additionally the precursors can be designed through variation of multiple parameters, such as "pH, precipitation temperature and atmosphere, sources of transition metals and their concentration, use of chelating agents and their concentration, rate of reactant feed, stirring rate and mixing method, drying temperature, and the use of any other additives"^[7].

Effect of synthesis parameters

Precipitation time

The morphology of the precursor is greatly affected by the precipitation time, which is synonymous with reaction time. A longer reaction time results in the structure being more ordered hexagonal, improvement in the crystallinity and therefore higher capacity retention and electrochemical performance in general. The longer the reaction time the smoother and more spherical the particles become. Also the size distribution decreases.^[7]

Reaction temperature

The temperature affects the solubility of the transition metals and how homogeneous the mixture is. Additionally to high temperatures can lead to more impurities and therefore decreasing integrity of the particles. However, different coprecipitation methods or stoichiometry of the metal salts lead to different effects  by temperature changes, which is why it should always be individually investigated.^[7]

pH

The pH can affect the properties of the precursor, especially Ni_xMn_yCo_1-x(OH)₂ in hydroxide precipitation. It controls the growth, nucleation and what species form in solution.  This way the particle size distribution, particle size and shape and therefore the tap density and energy density are affected. The more spherical particles are the higher the tap density.^[7] Generally speaking a higher pH leads to a decrease in the particle size. At around pH=11 the tap density reaches a maximum and the secondary particles are spherical.^[7]

Chelating agent

The chelating agent is used to prevent the formation multiple phases of the precursor and transition metals. This way phase separation is prevented and dense homogeneous precursors (OH-coprecipitation)  are formed. Ammonium hydroxide is most commonly used as a chelating agent in OH- and carbonate-coprecipitation,  its concentration greatly affects the tap density since it affects the uniformity and size distribution of the particles.^[7] The higher the concentration the higher the tap density, up until a maximum is reached.

Stirring rate

Finding the right stirring rate during the process of coprecipitation is also crucial. Agglomeration of primary particles during coprecipitation happens through the collision of particles among each other and the reactor wall. At low stirring rates (400 rpm) the size distribution is high and the tap density low. Increasing the stirring rate up to 1000 rpm leads to uniform, spherical secondary particles. However, increasing the rate higher leads to the cracking of the secondary particles and should be prevented.^[7]

Structure

The structure of Li-NMC is dependent on multiple parameters such as the stoichiometry of the precursors, the synthesis method and the synthesis parameters. However, the Li-NMC has a layered structure. The structure follows the  α-NaFeO₂ hexagonal structure, which crystallizes in the space group R-3m.^[8] The crystal structure of LiNi_xMn_yCo_zO₂  (x,y,z=1/3) can be found in Figure 2, where oxygen is red, lithium is green, cobalt is blue, nickel is silver and manganese is pink. The unit cell is shown on the left without coordination polyhedra and on the right with them. For different stoichiometry the spheres indicating the transition metals will be colored accordingly. The crystal structure information can be found in detail in table 1. 

Figure 2: The crystal structure and coordination polyhedra of LiNi_xMn_yCo_zO₂  (x,y,z=1/3).  Red= oxygen, green= lithium, blue=cobalt, silver= nickel, pink=manganese. The figure was created based on Yin^[9]using VESTA^[10]by Freya Huck.

Table 1:  Summary of crystal structure of LiNi_xMn_yCo_zO₂ (x,y,z=1/3) with data taken from Yin et al. ^[9].

Advantages & Disadvantages

Main advantage is the reduced cost  of Li-NMC batteries compared to LiCoO₂ (LCO) batteries through the reduction of the cobalt content. Additionally Li-NMC-batteries have a high energy density, high capacities (sometimes higher than for LCO) at similar operation voltage. As mentioned before the stoichiometry can be readily adjusted which makes the material design variable. Because of the high energy density, the batteries are smaller and lighter than their competitors (LFP). However one of the main disadvantages compared to LFPs is that Li-NMCs have a lower thermal stability, making them less safe.^[3]

Applications

Li-NMC batteries are mainly used in electric vehicles on the western market because of their advantages. The main reason is the high energy density and good life span. In China different materials are used for batteries in vehicles (e.g. lithium iron phosphate (LFP)).^[3] Li-NMC prismatic batteries with graphite as an anode are used in VW e-Golf. As a pouch cell they are used in the Chevrolet Bolt and Renault Zoe. In Tesla's utility storage products NMCs are used as well.^[3]

References