Enhance cycling performance of LiMn 2 O 4 cathode by Sr 2+ and Cr 3+ doping

Preparation for lithium ion secondary battery by Sr²⁺ and Cr³⁺ doping

The LSCMO powder was prepared by solid state method, Li(CH₂COO).2H₂O [analytical reagent (AR) pure], Mn(CH₂COO) .4H₂O (AR pure), Sr(NO₃)₂ (AR pure) and Cr₂O₃ (AR pure) were mixed into ball mill at the mole ratio of Li/Mn/Si/Cr (1·1∶1·8∶0·1∶0·1) (extra lithium was added for its consumption in preparation by the solid state method). The mixture was preheated for 4 h at 450°C at a heating rate of 2°C min⁻¹ within an atmosphere furnace, followed by reheating at 750°C for 40 h at a heating rate of 4°C min⁻¹ to get pure LSCMO powder.

Physical characterisation

X-ray diffraction (XRD) was performed on a DX-2700 X-ray diffractometer with Cu K_α radiation sources (1·5406 Å). X ray diffraction measurements were carried out by step scanning with an angular range of 10–90° and a step width of 0·03°. The chemical composition of the powder was analysed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The particle morphologies were examined using an EVO-18 SEM with a microscope magnification of ×10 000 at a voltage of 20·0 kV. Electron diffraction spectroscopy (EDS) was applied to determine the elements of powders together with SEM in large field of view.

Electrochemical characterisation

The electrochemical measurements were carried out on half cells. A mixture of 80 wt- of each active material and 10 wt- of acetylene black was added to an N-methyl-2-pyrrolidene solution containing 10 wt- of polyvinylidene fluoride. The slurry was then coated on an aluminium current collector. Then, the slurry was dried at 80°C for 18 h in a vacuum chamber. The coated cathode foil was pressed and cut into circular sheets of 16 mm diameter.

The experiment cells were packaged in an Ar filled glovebox. The cell comprises a positive electrode, a diaphragm Celgard2300 as the separator, a piece of lithium plate as anode electrode and 1M LiPF₆ dissolved in a mixture of ethylene carbonate and dimethyl carbonate (volume ratio of 1∶1) as the electrolyte. The experiment cells were charge–discharge on a battery tester (CT3008W-5V 5mA-S 4, china) between 3·00 and 4·35 V(Li/Li⁺) with a current density of 1, 5, 10 and 20 C. Cyclic voltammetry (CV) studies were carried out at a scan rate of 0·10 mV s⁻¹ between 3·20 V and a maximum value of 4·40 V using electrochemical workstation (CHI600D).

Analysis of XRD

Figure 1 shows the XRD patterns of LSCMO, LMO mixed Sr(NO₃)₂, Cr₂O₃, Cr₂O₃ and Sr(NO₃)₂. The LSCMO powder has a well defined spinel structure with a space group of Fd3m.^46,52 It is found that the diffraction peaks at 2θ = 19·22, 36·75, 37·88, 44·79, 48·45, 58·16, 64·76 and 68·89° are relative to the crystal planes of (1 1 1), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (5 1 1), (4 4 0) and (5 3 1). No impurity is detected. Compared with the XRD of LSCMO and LMO mixed Cr₂O₃ and Sr(NO₃)₂. It is clear that LSCMO is significantly different from LMO mixed Cr₂O₃ and Sr(NO₃)₂. It is indicated that doping is different from mixture. The lattice parameters and ICP-AES data of LMO and LSCMO are summarised in Table 1. The full width at half maximum (FWHM) values of the (400) diffraction lines of LMO decline with Sr²⁺ and Cr³⁺ doping. This indicates that the LSCMO might have higher crystallinity, better ordering of local structure and lower lattice strain. ⁵³ The compositions of the LMO and LSCMO were determined to be Li_1·12Mn_1·89O_3·98 and Li_1·09Sr_0·09Cr_0·11Mn_1·88O_3·94.

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X-ray diffraction patterns of samples

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Table 1

Average lattice constants and FWHMs at (400)

Group	LMO	LSCMO
Lattice parameter/Å	8·241	8·237
Unit cell volume/Å3	559·6	558·8
FWHM 400/°	0·1640	0·1610
Composition through ICP-AES	Li1·12Mn1·89O3·98	Li1·09Sr0·09Cr0·11Mn1·88O3·94

Scanning electron microscopy and EDS

The morphology of samples was investigated using SEM and EDS. It can be seen that the LMO powders are in the order of 200–1000 nm from Fig 2a. Figure 2b and c showed the SEM of LiCr_0·1Mn_1·9O₄ and LiSr_0·25Mn_1·75O₄. Thirunakaran found that, at lower chromium concentrations (x = 0·10 and 0·20), the particles remain as fine of average grain size below 500 nm. Subramania explained that product is porous in nature. This feature facilitates easy access of battery electrolyte and thereby increases the number of active sites, which is desirable for good battery activity. ⁵⁴ The particles of LSCMO show irregular shape and are uneven from 800 to 2000 nm (Fig. 2d). The diameter of particle size increases after Sr²⁺ and Cr³⁺ doping. In the EDS image (Fig. 2e), Au element is detected because the sample was sprayed gold during processing. There are Sr and Cr elements in addition to the Li, Mn and O signals. This indicated that Sr and Cr element exist as part of LMO morphology.

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a–d images (SEM) of samples (a LMO, b LiCr_0·1Mn_1·9O₄, c LiSr_0·25Mn_1·75O₄, ⁵⁴ d LSCMO) and e EDS of LSCMO powder

Cyclic voltammetry study

Figure 3 displays the CV of the LMO and LSCMO electrodes in the potential range of 3·20–4·40 V(Li/Li⁺) at a scan rate of 0·10 mV s⁻¹. Two pairs of redox peaks appear in CVs, which locate at 4·14/4·01 V and 4·02/3·82 V for LSCMO electrode and 4·20/4·02 V and 4·04/3·81 V for LMO electrode. It is known that these two pairs of current peaks originate from two reversible phase–phase transitions of LMO during intercalation and deintercalation processes. ³² Intercalation and deintercalation of Li⁺ detected great growth in peak current after doping, which promotes the high current charge–discharge performance for the battery. The chemical diffusion coefficient of Li⁺ ion is an important factor to influence the kinetic of the electrode materials. ⁵⁵ The can be calculated from equation (1) ⁵⁶ (1) where I^P, z, A, and C⁰ are the peak current (A), the charge transfer number, the electrode area (cm²), the chemical diffusion coefficient (cm² s⁻¹) and the bulk concentration of Li⁺ ions. ³² We can conclude that the of LSCMO is higher than of LMO. The results implied that the sample of LSCMO would have a better rate capability than LMO. ³¹

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Cyclic voltammograms of LMO and LSCMO electrodes in potential range of 3·20–4·40 V [E versus Li (V)] at a scan rate of 0·10 mV s⁻¹

Analysis of charging–discharging and cyclic performance

The electrochemical properties of samples were systematically investigated using their charge–discharge capacity and cycling stability. Figure 4a displays the initial charge–discharge curves of LSCMO between 3·00 and 4·35 V at currents of 1, 5, 10 and 20 C. The charge–discharge curves showed the discharge capacity is 128 mA h g⁻¹ at 1 C and still remains 100 mA h g⁻¹ at 20 C. The LiCr_0·04Mn_1·96O₄ cell showed that the discharge capacity is 122 mA h g⁻¹ at 0·5 C and remains 112 mA h g⁻¹ after 50 cycles. ³⁶ Figure 4b displays the initial charge–discharge curves of LMO between 3·00 and 4·35 V at currents of 1, 5 and 10 C. The charge–discharge curves showed that the discharge capacity is 122 mA h g⁻¹ at 1 C and still remains 46 mA h g⁻¹ at 10 C. Compared with Fig. 4a, it demonstrates that the large current charge–discharge performance of LMO was improved by Sr²⁺ and Cr³⁺ doping. We examine the cyclic performance of half cells with LSCMO under current densities of 1, 5, 10 and 20 C (Fig. 4c). For LSCMO, the discharge capacity declined from 128 to 116 mA h g⁻¹ at 1 C, with a capacity loss of 93·75 after 100 cycles. The capacity retention ratios after 100 cycles of LSCMO are 90·18 at 5 C, 86·95 at 10 C and 78·96 at 20 C. Figure 4d shows that cyclic performance of half cells with LMO under current densities of 1, 5 and 10 C. The capacity retention ratios after 100 cycles of LMO are 76·66 at 1 C, 43·95 at 5 C and 26·66 at 10 C. The cyclic performance of the LMO was improved by Sr²⁺ and Cr³⁺ doping.

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a initial charge–discharge curves of LSCMO; b initial charge–discharge curves of LMO; c cyclic performance of LSCMO; d cyclic performance of LMO