Influence of Core and Shell Properties in Core-Shell Positive Electrode Materials for Li Ion Batteries

Introduction

Core-shell materials with a Ni-rich core and a Mn-rich shell are possible next generation positive electrodes for Li-ion cells.  A Ni-rich core can deliver high energy density and a Mn-rich shell can minimize the electrolyte oxidation.  For example, Jing Li et al. have recently developed core-shell material that exhibits high reversible capacity, low irreversible capacity loss as well as reduced voltage fade¹. This study examines the contribution from core and shell materials to the resultant electrochemical properties of the core-shell material.

Synthesis

Core-shell transition metal carbonate precursors were made using coprecipitation in a continuously stirred tank reactor (CSTR). Appropriate temperature and pH conditions were established for the core and shell precipitations. The core was precipitated first and then shell was precipitated subsequently on the surface of the core. Then appropriate amounts of Li₂CO₃were mixed with the core-shell precursors and fired at high temperature to yield the final product.

Results and Discussion

The compositions of the materials selected for the core or shell were: A - Ni_0.166Mn_0.5Co_0.333CO₃ and H - Ni_0.4Mn_0.5Co_0.1CO₃. Two core-shell precursors (CS-AH and CS-HA) with 80:20 core to shell ratio were synthesized using A and H.  In CS-AH, precursor A is the core and H is the shell whereas in CS-HA the core and shell compositions were interchanged.  Figures 1a and 1c show SEM images of CS-AH and CS-HA respectively and Figures 1b and 1d show cross-sectional EDS elemental maps. In Figures 1b and 1d, the bluish-red and bluish-green regions indicate the Co-rich and Ni-rich phases.

LiMO₂type materials were synthesized from A, H, CS-AH and CS-HA precursors and they were labelled as A2, H1, CS-A2H1 and CS-H1A2. Figure 2 shows the first cycle charge-discharge profiles of the samples in coin type cells. Sample A2, which was synthesized by following reference 2, exhibits a relatively low IRC (9.25%) compared to sample H1 (20.6%). For sample CS-A2H1, which has 80% A2 core and 20% H1 shell, the IRC was 10.8%.  This IRC value is approximately the linear combination of the individual IRCs obtained from A2 and H1. Similarly, sample CS-H1A2, that has 80% H1 core and 20% A2 shell exhibits an IRC of 17.7%.

Conclusion

Core-shell positive electrode materials are promising due to the complementary benefits from the core and shell properties. Two core-shell type positive electrode materials with core and shell compositions interchanged from one another were synthesized and compared with core and shell only materials. The results suggest that the IRC of core-shell materials are simply the linear combination of those of the core and shell only values. Other electrochemical results, in particular, ultra high precision coulometry, to probe the impact of the shell phase on electrolyte oxidation, will be discussed in detail.

References:

1. Li, Jing; Camardese, John; Shunmugasundaram, Ramesh; Glazier, Stephen; Lu, Zhonghua; Dahn, J R, accepted for publication in Chemistry of  Materials
2. Shunmugasundaram, Ramesh; Senthil Arumugam, Rajalakshmi; Dahn J R, Chem. Mater., 2015, 27 (3), pp 757–767