1. XRD patterns (a) and the second charge-discharge curves at 100 mA/g (b) of the products heat-treated at various temperatures with Cr³⁺ content of 0.4

2. XRD patterns of the xerogels with Li_1+xCr_xSn_2-x(PO₄)₃ (x=0-0.5) heat-treated at different temperatures(b) Enlarged patterns of (a)

3. Second charge-discharge curves at 100 mA/g (a) and the discharge capacities at different current densities (b) of the batteries assembled with the samples containing different Cr³⁺ contents, and the charge-discharge curves at different rates for the samples of Li₁Sn₂(PO₄)₃ (c) and Li_1.3Cr_0.3Sn_1.7(PO₄)₃ (d)

4. Cyclic charge-discharge curves at 100 mA/g of the batteries assembled with the sample Li_1.3Cr_0.3Sn_1.7(PO₄)₃ (a), variation in discharging capacity for the samples of x=0 and x=0.3 (b) and rate performance between 100 mA/g and 800 mA/g for the batteries assembled with the Li_1.3Cr_0.3Sn_1.7(PO₄)₃ sample (c)

5. AC impedance spectra (a) and relationship between Z° and ω^-1/2of the samples with different Cr³⁺ contents (b) with inserts showing local magnification of AC impedance spectra and their equivalent circuit

S1. SEM images and EDX patterns of the Li_1+xCr_xSn_2-x(PO₄)₃ samples

S2. XPS spectra for the Li_1+xCr_xSn_2-x(PO₄)₃ samples with x of 0.2, 0.3 and 0.4

S3. XRD patterns of the Li_1.3Cr_0.3Sn_1.7(PO₄)₃ electrode after charging and discharging at different voltages (a), and enlargement (b) of the box region in (a)