Résumé
Li-ion batteries have contributed to the commercial success of portable electronics and may soon dominate the electric transportation market provided that major scientific advances including new materials and concepts are developed. Layered oxides of the general formula LiNi
1/3
Co
1/3
Mn
1/3
O
2
, termed as NMC, display the highest capacity (≈ 200 mAh/g) of any presently used positive electrode materials.
1
By modifying the chemical composition through substitution of Li for M in the metal layers, Thackeray’s and Dahn’s groups have confectioned materials displaying reversible capacities exceeding 250 mAh/g.
2-3
However, their implementation in practical Li-ion cells is still plagued by the complex electrochemical mechanism and large voltage decay upon cycling. Elucidating the origin of large capacity in these materials and the cause of voltage decay so that a practical solution can be identified remains two fundamental challenges addressed by numerous groups worldwide.
In an attempt to understand both the aforementioned issues, we have designed structurally related Li
2
Ru
1-y
M
y
O
3
materials (M= Sn, Mn, Ti; 0 ≤ y ≤ 1).
4-6
Containing a single redox-active cation and displaying sustainable reversible capacities as high as 230 mAh/g, the Li
2
Ru
1-y
Sn
y
O
3
phases turn out to be the ideal system to unambiguously show, based on an arsenal of characterization techniques, that the reactivity of these high capacity materials towards Li entails cumulative cationic (M
n+
→M
(n+1)+
) and anionic (“O
2–
→O
2
2–
”) reversible redox processes.
4
To address the voltage decay issue, Ti
4+
(d
0
) substitution was selected owing to its zero CFSE ,similar to Sn
4+
(d
10
), and its smaller size (0.60Å); with the expectation that Ti would be more likely to show accelerated cation migration for direct visualization of migration paths.
6
The results of such studies will be reported with the hope that they will help the battery community to develop high capacity layered electrodes free from voltage decays upon cycling.
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