Lithium –Manganese Rich Li1.2Mn0.55Ni0.15Co0.1O2 (LMR-NMC) Based Cathodes and Si-C Composite Anodes for High Energy Density Lithium-Ion Batteries
Authors: Martha Surendra Kumar , Sarode Krishnakumar , Ghosh Sourav , Bhar Madhushri 
Published: February 2020

US Department of Energy predicts the next generation lithium ion batteries (LIBs) will have almost the double the energy density (235 Wh kg-1) of the currently available Li-ion batteries based on layered transition metal (TM) oxides or Phosphates, Mn based spinels as cathodes and graphitic carbon as the anode. Besides the batteries should have reduced cost (100-125 USD per kWh) and improved thermal safety. The nominal capacity of most of these cathodes are in the range between 140-180 mAhg-1 when cycled up to 4.2V. This is only half the specific capacity of graphite anode (372 mAhg-1). Thus, there has been an intense research activity in the last decade to develop high capacity or high energy cathodes and anodes for lithium-ion batteries. Lithium and manganese rich TM oxide composite cathodes such as, xLi2MnO3. (1-x)LiMO2 (M= Co, Mn and Ni, etc.) (LMR-NMC) that has almost double the capacity (280 mAh g-1) of layered TM oxides, but needs to be electrochemically cycled to a voltage > 4.5 V which causes electrochemical instability. When cycled above 4.5 V the Li2MnO3 component is activated and forms Li2O and MnO2. The decomposition contributes to the high irreversible capacities >25% for LMR-NMC electrodes and capacity fade due to significant reduction of electrolytes and electrode conductivity due to high voltage cycling [3]. The capacity fade also associated with Mn dissolution from the host structure upon repeated high voltage cycling. Beside, LMR-NMC cathodes undergo structural changes upon high voltage cycling causing energy loss. The energy density of LMR-NMC cathodes are reduced from 1000 Wh kg-1 during initial cycles to ~750 Wh kg-1 during 100th cycle which makes LMR-NMC practically impracticable. The structural transition to the spinel phase is an intrinsic phenomena associated with Ni ions migration to the lithium layer during high voltage cycling.

In this work we present doped or blended composite electrodes to reduce energy loss in LMR-NMC during high voltage cycling [1-2]. LiF coating onto LMR-NMC is used to stabilize thes interface and mitigate voltage fade [1]. In this approach, some of the M-O bonds are replaced by M-F bonds on the surface. The M-F bond is stronger and stabilizes the interphase during cycling. Partially, O2- is replaced by F- on the surface of LMR NMC due to which the average oxidation state of the surface metal ions is slightly decreased which lead to decrease in charge potential, thus minimizing the electrolyte decomposition and delivering better electrochemical performance. Fluorine doped cathodes deliver high capacity of ~300 mAh g-1 at C/10 rate, have high discharge voltage plateau (> 0.25V) and low charge voltage plateau (0.2 to 0.4V) compared to pristine LMR NMC cathodes.

Beside fluorine doping, improved interfacial stability and reduce voltage decay can be achieved through both cation and anion dopings. By cation doping of Ni with Mg, voltage fade has been significantly improved due to structural stabilization. By substituting Ni2+ with Mg2+ can minimize the cation migration as it blocks the tetrahedral void through which movement of cations takes place from transitional metal layer to Li-layer. The synergistic effect of both magnesium and fluorine substitution on electrochemical performance of LMR-NMC shows excellent discharge capacity of ~300 mAhg-1 at C/20 rate and cycles well. Besides, blending with LiMnPO4
has been found a classical strategy to improve interfacial stability and voltage drop in LMR-NMC [2].

Silicon is an attractive anode material for Li-ion batteries mainly because of its very high theoretical charge capacity of 4200 mAh g-1 (Li4.4Si) and natural abundance. But Si undergoes 310% volume change during lithium insertion, causing pulverization and subsequently rapid capacity fade. To enhance the performance of Si-C anodes, nanoengineering and 3D electrode architecture of Si-C electrodes developed shows >2000 mAh g-1 capacity for the Si-C electrodes with >250 cycles. Full cells developed using LMR-NMC cathodes and Si-anodes in Li-ion coin type cells shows high open-circuit voltage of >4V and energy density of >500 Wh kg−1 (Fig.1), more than the double energy of the currently available LIBs [3]. It is believed that the study opens a new realm of possibility for the development of high energy density Li-ion batteries that could be ideal for electric vehicles.

Acknowledgements

This work has been supported by IIT Hyderabad and DST-SERB under project code: SB/FT/CS-147/2014.

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Figure 1