In commercial Li-ion batteries, positive electrode materials such as LiFePO₄ or LiCoO₂ are based on only one electron exchange reaction (and consequently one Li⁺) per metal atom. The Cr-based materials present stable oxidation states from 2+ to 6+. Thus, in theory, it is possible to overcome the one-electron limitation and to obtain high specific charge with Cr-based polyanion materials Li_yCr_zX (X as anionic part). However, very few relevant electrochemical investigations have been reported so far [1].

The material LiCrP₂O₇ has been chosen as reference for the proof of concept, since the synthesis [2] and crystal structure [3] are already described in the literature. The supposed Cr^3+/2+ redox couple could be experimentally confirmed by us already, but no proof was found for the Cr⁴⁺oxidation state [4].

Thus our investigations were directed to the Li₃Cr₂(PO₄)₃ material, where the synthesis and crystal structure [5] were already been described in the literature. Furthermore, a recent publication showed the possibility of a reversible formation of Cr⁴⁺ in Li₃Cr₂(PO₄)₃, at least on the surface (as shown by XPS investigation) at high potential 4.5-4.9 V vs. Li⁺/Li [1]. This additional redox activity makes Li₃Cr₂(PO₄)₃ a promising cathode material.

Li₃Cr₂(PO₄)₃ was synthesized via sol-gel method as well as solid state route with calcination under inert atmosphere. The obtained powder was characterized by neutron powder diffraction and electrochemically tested with a coin-cell set-up.

In Figure 1, the electrochemical signature between 1.5–2.5 V vs. Li⁺/Li is attributed to the Cr³⁺/Cr²⁺ redox couple [6]. At higher potentials, we observe oxidation and reduction peaks at 4.83 V and 4.73 V vs. Li⁺/Li, respectively for the solid state carbon coated material. These oxidation/reduction peaks are attributed to the redox couple Cr³⁺/Cr⁴⁺ which is consistent with the DFT calculations done by Hautier et al [6].

In galvanostatic cycling at C/10 rate, Li₃Cr₂(PO₄)₃ (sol-gel) shows an initial specific charge of 150 mAh/g. It is superior to the theoretical value of 130.8 mAh/g when only 1 Li⁺ is exchanged per Cr. The initial specific charge obtained from cycling between 1.5-4.9 V vs. Li⁺/Li corresponds to a 1.15 Li⁺-exchange per formula unit. The specific charge is then stabilizing at ca. 110 mAh/g after 10 cycles.

Operando X-ray diffraction measurements show a reversible change in the crystal structure upon cycling. The shift of the (111)-reflection is following the lithiation and delithiation of the structure, leading to the assumption of an insertion mechanism. This behavior was followed for two complete cycles, which confirms the reversibility of the assumed lithium insertion/deinsertion reaction mechanism.

To further extend this concept of new high energy density materials for Li-ion batteries, long-term cycling results of full cells will be shown, first with standard graphite and second with Fe_0.5TiOPO₄ titaniumoxyphosphates (specific charge > 700 mAh/g) selected as anode and Li₃Cr₂(PO₄)₃ as cathode, respectively.

References

[1]           M. Herklotz, F. Scheiba, R. Glaum, E. Mosymow, S. Oswald, J. Eckert, H. Ehrenberg, Electrochimica Acta 2014, 139, 356–364.

[2]           Gangulibabu, D. Bhuvaneswari, N. Kalaiselvi, Applied Physics A 2009, 96, 489–493.

[3]           L. S. Ivashkevich, K. A. Selevich, A. I. Lesnikovich, A. F. Selevich, Acta Crystallographica Section E 2007, 63, i70–i72.

[4]           M. Reichardt, C. Villevieille, P. Novak, S. Sallard, Acta Crystallographica Section B 2015.

[5]           J. Sun, P. Kim, H. Yun, Acta Crystallographica Section E 2013, 69, i72.

[6]           G. Hautier, A. Jain, S. P. Ong, B. Kang, C. Moore, R. Doe, G. Ceder, Chemistry of Materials 2011, 23, 3495–3508.