C/Li2MnSiO4 Nanocomposite Cathode Material for Li-Ion Batteries

1. Introduction

Technological development of portable devices, e.g, mobile phones, laptops, etc., as well as progress in electrical vehicles (EV) and hybrid electrical vehicles (HEV) technologies require batteries efficient in volumetric and gravimetric energy storage, exhibiting large number of charge/discharge cycles and being cheap and safe for users. Moreover, materials used in energy storage and conversion systems should be environmentally friendly and recyclable. Currently, rechargeable lithium-ion batteries (LIBs) are the most popular portable energy storage system, mostly due to their highest energy density among all others rechargeable battery technologies, like Ni-Cd or Ni-MH cells which reached the theoretical limit of performance. Commercially available LIBs are based on layered lithium cobalt oxide (LiCoO₂) or related systems, which are expensive and toxic. These materials are unstable in an overcharged state, thus the battery safety is affected, especially in high power (20-100 kWh) applications for EV, HEV and renewable energy systems. The bigger battery capacity results in more energy accumulated, thus operational safety is a key issue. On the other hand, lifetime and capacity retention of LIBs in changeable operation conditions (from -30°C to +60°C, average lifetime 2-4 years) are a challenges to develop new materials and cell assembly technologies.

2. Li-ion battery technology

First rechargeable lithium cells taking advantages of intercalation process were developed in year 1972 [1]. The cells Li/Li⁺/Li_xTiS₂ revealed 2V potential and relatively low gravimetric capacity. Applications of metallic lithium as anode material resulted in common cell breakdown due to formation of dendritic structures on anode during cell cycling. The problems forced research and development of new intercalation materials for lithium batteries. In the eighties a new conception of lithium cell was proposed (so called Li-ion batteries or “rocking-chair batteries”), which consisted of application of two different lithium intercalation compounds as anode and cathode materials [2-4]. As anode material a graphite intercalated with lithium was used while cathode materials were based on layered 3d transition metal oxides.

2.1. Layered LiCoO₂ oxide and related systems

The first Li_xC₆/Li⁺/Li_1-xCoO₂ Li-ion battery system was commercialized in 1993 by Sony Co. In Fig. 1 a working mechanism during discharge cycle of Li-ion cell is presented.

Figure 1.

Working mechanism of Li-ion batteries (discharge cycle).

The electrochemical reaction at the graphite anode side can be written as (1):

$${\text{C}}_{\text{6}}+{\text{ xLi}}^{+}+{\text{ xe}}^{-}\leftrightarrow {\text{Li}}_{\text{x}}{\text{C}}_{\text{6}}$$E1

and suitably at LiCoO₂ cathode side (2):

$${\text{LiCoO}}_{\text{2}}\leftrightarrow {\text{Li}}_{\text{1}-\text{x}}{\text{CoO}}_{\text{2}}+{\text{ xLi}}^{+}+{\text{ xe}}^{-}$$E2

Commercially available LIBs based on layered lithium cobalt oxide (LiCoO₂) or related systems (LiNi_yCo_1-yO₂, LiMn_yCo_1-yO₂, LiMn_1/3Ni_1/3Co_1/3O₂) reveal reversible capacity 130-150 mAh/g and working potential 3.6-3.7 V. The oxide materials are expensive and toxic due to cobalt content and are unstable in an overcharged state, thus the safety is strongly affected. This is related to strong oxidizing behavior of the charged layered oxide cathode in contact with organic electrolyte what may lead to combustion or even explosion [4, 5]. Unfortunately, this effect may be also increased by application of nanosized materials with high surface area.

2.2. Spinel LiMn₂O₄ and related systems

An alternative material for cathode based on layered oxides LiM_yCo_1-yO₂ (M=Ni, Mn, Fe) is LiMn₂O₄ spinel. LiMn₂O₄ reveals a little lower reversible capacity 120 mAh/g at working potential 4 V, but the material is distinctly cheaper and nontoxic. However, application of spinel as cathode material in commercial Li-ion batteries is retarded by phase transition observed near room temperature, i.e. battery operation temperature, and related to Jahn-Teller distortion of Mn³⁺ ions, what resulted in capacity fading. Stabilization of cubic spinel structure is possible by controlled formation of cationic defects [6], by lithium [7] or 3d metal substitution [8] into spinel lattice as well as by isoelectronic sulfur substitution [9, 10]. Instability of high oxidation state of transition metal in spinel structure observed in charged state of cathode leads to oxygen evolution, similarly to the layered oxide cathodes, and reaction with electrolyte. Application of spinel based materials is limited to cheap battery packs for EV [11].

2.3. Olivine LiFePO₄ cathode material

An interesting and very promising group of insertion materials are LiMXO₄ (M= metal 3d, X=S, P, As, Mo, W) type compounds [12], with LiFePO₄ among them. LiFePO₄ lithium iron phosphate of olivine structure is chemically and thermally stable material with relatively high gravimetric capacity 170 mAh/g at working potential 3.5 V. The material is cheap, nontoxic and environmental friendly. Its high chemical stability towards electrolyte, related to strong P-O bonds, significantly improves safety of LIBs. However, very low electrical conductivity of LiFePO₄ system (10^-9 S/cm @RT) requires application of carbon coatings and composite formation [13, 14].

3. Orthosilicates Li₂MSiO₄ – New high capacity cathode materials

Application of orthosilicates Li₂MSiO₄ (M=Fe, Mn, Co, Ni) compounds as insertion materials for LIBs was firstly proposed by Prof. Goodenough [12, 15]. The materials mainly crystallizes in orthorhombic system of Pmn21 space group in olivine structure [16-18]. The Li₂MSiO₄ olivine structure may be described as wavy layers of [SiMO₄]_∞ on ac axis plane and connected along b axis by LiO₄ tetrahedra [16]. Within the layers every tetrahedron SiO₄ shares its corner with four next MO₄ tetrahedra. Lithium ions occupy the tetrahedral sites (LiO₄) between two layers and share 3 and 1 oxygen atoms with the layers. In fact, diffusion of lithium ions in this structure is possible only through the canals formed by LiO₄ tetrahedra. Li₂MSiO₄ silicates reveal a possibility of reversible insertion of two lithium ions per molecule, so this leads to exchange of two electrons by transition metal. As a results the silicates reveal very high theoretical gravimetric capacity, up to 330 mA/g. However, structural limitation due to sharing of LiO₄ between two wavy layers, should affect a little the capacity. Calculations of electrochemical potential of lithium insertion/deinsertion show a two stage process:

$${\text{Li}}_{\text{2}}{\text{M}}^{\text{2}+}{\text{SiO}}_{\text{4}}\to \Upsilon {\text{LiM}}^{\text{3}+}{\text{SiO}}_{\text{4}}+{\text{ Li}}^{+}$$E3

$$\Upsilon {\text{LiM}}^{\text{3}+}{\text{SiO}}_{\text{4}}\to {\Upsilon }_{\text{2}}{\text{M}}^{\text{4}+}{\text{SiO}}_{\text{4}}+{\text{ Li}}^{+}$$E4

Depending on transition metal potential of the reaction (3) vary from 3.2 V (for Fe) to 4.1-4.4 V (for Mn, Co, Ni), while the potential of reaction (4) is in range 4.5-5.0 V. Deinsertion of the second lithium ion requires applying high potential above 4.5 V, and this is a challenge for electrolyte. Orthosilicates Li₂MSiO₄ (M=Fe, Mn, Co, Ni) materials reveal very low electrical conductivity (10^-12 - 10^-15 S/cm @RT) and carbon coating of the materials is required. On the other hand, downsizing of material grains should improve electrochemical performance [18]. Very strong covalent bonding of Si-O results in high chemical stability towards electrolyte, strongly increasing the safety of LIBs based on silicates. The materials are nontoxic, environmental friendly and cheap. The properties mentioned above provide a challenge for developing a composite cathode materials based on Li₂MSiO₄ orthosilicates.

4. Li₂MnSiO₄ nanostructured cathode material

Li₂MnSiO₄ is a member of dilithiumorthosilicates Li₂MSiO₄ (M = Fe, Mn, Co) family, thanks to strong covalent Si-O bond, it shows high thermal and chemical stability. High theoretical capacity 333 mAh/g, low production costs, safety and optimal working potential make them an attractive cathode material. Very low electrical conductivity (10^-12 - 10^-15 S/cm @RT) can be improved by coating with conductive carbon layers (CCL) and by grains size downsizing [18-20]. The properties of Li₂MnSiO₄ material comes to a conclusion that cathode material for LIBs based on this compound should be prepared as C/Li₂MnSiO₄ nanocomposite. Thus, the special preparation techniques have to be applied in terms to obtain nanocrystaline grains of Li₂MnSiO₄ cathode material coated by conductive carbon layers (CCL). During last few years several different technics of Li₂MnSiO₄ synthesis were proposed. Hydrothermal synthesis [21-24] as well as solid-state reactions [25-28] can lead to one phase product but the control of the grain size is significantly limited. Sol-gel method is one of the soft chemistry techniques which can be used in synthesis of nanosized lithium orthosilicates [29-33]. Sol-gel processes, especially Pechini’s method is a simple technic, characterized by low cost and low temperature of treatment, resulting in homogenous, high purity materials.

4.1. Preparation of nanostructured Li₂MnSiO₄

Li₂MnSiO₄ was produced using sol-gel synthesis – Pechini type. Starting reagents were: lithium acetate dihydrate (Aldrich), manganese acetate tetrahydrate (Aldrich), tetraethoxysilane (TEOS) (98%, Aldrich) as a source of silicon, ethylene glycol (POCh), citric acid (POCh) and ethanol (POCh). Thanks to chelating metal ions in solution by citric acid the cations can be mixed at the molecular level and the stoichiometric composition can be achived. The reactants were mixed in a molar ratio 1:1:18:6:4:16 - Mn:Si:C₂H₆O₂:C₆H₈O₇:C₂H₅OH:H₂O. Based on previous studies it was affirmed, that using 20% excess of lithium acetate leads to one-phase product. All reagents were dissolved in glass reactor under constant argon flow (Ar 5.7). As a solvent, only stoichiometric amount of distilled water was used. Heating water to 35 ˚C assure fast and complete dissolution of metal acetates. Prepared mixture was heated to 60 ˚C and few drops of concentrated hydrochloric acid was added to initiate polymerization of metal citrates using ethylene glycol and TEOS. Reaction was conducted for 24h in close reactor. Obtained gel was aged for 3 days at 60 ˚C in close reactor (Ar atmosphere) and for 3 days at 60 ˚C in an air-drier (Air atmosphere).

Thermogravimetric analysis (TGA) coupled with simultaneous differential thermal analysis (SDTA) and mass spectrometry evolved gas analysis (MS-EGA) of precursor were performed in Mettler-Toledo 851^e thermo-analyzer using 150 μl corundum crucibles under flow of air/argon (80 ml/min), within temperature range 20–800 °C with heating rate of 10 °C/min. The simultaneous MS-EGA was performed in on-line joined quadruple mass spectrometer (QMS) (Thermostar-Balzers). The 17, 18 and 44 m/z mass lines, ascribed to OH, H₂O and CO₂ species respectively, were collected during the TGA experiments. TGA of the Li₂MnSiO₄ precursor are shown in the Fig. 2 and Fig. 3.

Figure 2.

Thermal analysis (TGA/DTG/SDTA) conducted under air atmosphere (a) with evolved gas analysis (EGA-QMS) (b) of Li₂MnSiO₄ precursor.

Figure 3.

Thermal analysis (TGA/DTG/SDTA) of Li₂MnSiO₄ precursor performed under inert atmosphere (Ar 5.0).

According to TGA curve (Fig. 2a) a complete decomposition of gel organic matrix occurs above 500 °C. Evolved gas analysis confirmed the disintegration of organic components. Amount of active material can be estimated at about 12 wt.% of the precursor. Basing on TGA results calcination conditions were chosen. Li₂MnSiO₄ precursor was calcined under Ar flow at 600, 700, 800 and 900 °C.

4.2. Properties of nanostructured Li₂MnSiO₄

X-ray powder diffraction patterns of the samples were collected on BRUKER D2 PHASER using Cu K_α radiation = 1.5418 Å. In Fig. 4 XRD patterns of Li₂MnSiO₄ obtained at different temperatures (600, 700, 800 and 900 °C) are collected.

Figure 4.

XRD patterns of Li₂MnSiO₄ after calcination at 600 °C, 700 °C, 800 °C and 900 °C. Li₂MnSiO₄ (Pmn2₁ phase) diffraction lines positions are marked in the top of the figure.

In the sample calcined at 600 °C all diffraction lines can be attributed to pure Li₂MnSiO₄ phase (Pmn2₁). In other specimens obtained at higher temperatures, MnO and Li₂SiO₃ impurities starts to appear. Formation of MnO and Li₂SiO₃ in samples calcined at temperatures above 700 °C can be connected, with Li₂MnSiO₄ decomposition. Average crystallites sizes calculated from diffraction lines broadening based on Scherrer equation are in the range of 17-19 nm depending on calcination temperature of the sample (Tab. 1). The calcination under carefully selected conditions leads to a transformation of a part of organic compounds from the gel matrix into thick carbon layer covering the active material. The temperature programmed oxidation (TPO) performed using TGA equipment confirmed presence of carbon in samples. Fig. 5b presents exemplary TG/DTG/SDTA curves from TPO conducted under air flow in temperature range of 20-1000 °C. The amounts of carbon in samples calcined in different temperatures were calculated from TPO measurements and are collected in Table 1.

Measurements of the specific surface area of the samples were performed in Micrometrics ASAP 2010 using BET isotherm method. About 500 mg of each sample was preliminary degassed at 250–300 °C for 3 h under pressure 0.26–0.4 Pa. Then, N₂ sorption was performed at pressure of 8 10⁴ Pa. One of pore size distribution plot with the adsorption-desorption curves inset is presented in Fig. 5a. Exact values of specific surface area and average pores diameters are presented in table 1.

Figure 5.

Adsorption-desorption isotherm and pore diameter distribution of C/Li₂MnSiO₄ calcined at 600°C, b) TGA/DTG/SDTA curves of C/Li₂MnSiO₄ calcined at 600 °C.

Low temperature N₂-adsorption (BET) measurements of C/Li₂MnSiO₄ composites show that specific surface area of samples decrease with increasing calcination temperature. Decrease of specific surface area is connected with graphitization of carbon during calcination at higher temperatures.

Sample name	Precursor calcination temperature	Carbon content	Specific surface area (BET)	Average pore dimension (BJH)	Crystallite size (from XRD)
C/Li2MnSiO4@600	600 °C	34%	169 m^2/g	50 Å	17 nm
C/Li2MnSiO4@700	700 °C	32%	152 m^2/g	72 Å	18 nm
C/Li2MnSiO4@800	800 °C	32%	144 m^2/g	73 Å	19 nm
C/Li2MnSiO4@900	900 °C	35%	57 m^2/g	73 Å	19 nm

Table 1.

Morphology parameters of C/Li₂MnSiO₄ composites calcined at different temperatures.

Electrical conductivity was measured using the AC (33Hz) 4-probe method within temperature range of -40÷55 °C. The carbon coated composite powders were so elastic that the standard preparation of pellets was impossible. The fine powder samples were placed into a glass tube and pressed by a screw-press between parallel gold disc electrodes (∅=5 mm) till the measured resistance remained constant. The results of electrical conductivity measurements are gather in Fig. 6. All composites exhibit good electrical conductivity (up to 0,36 S/cm for C/Li₂MnSiO₄@900) in comparison with Li₂MnSiO₄ itself ( ̴ 10^-15 S/cm at room temperature - RT). It can be observed that temperature dependence of conductivity for composites calcined at higher temperatures (700, 800 and 900 °C) seem unaffected, by temperature (metallic-like behavior). Conductivity value and its invariability against temperature indicate that carbon layers consist of graphite-like domains. In case of C/Li₂MnSiO₄@600°C the carbon layers is more disordered and probably consist of an activated-like carbon. Those results are consistent with BET measurements in case of graphitization process occurring at higher temperatures.

Figure 6.

Electrical conductivity measurement of C/Li₂MnSiO₄ (temp. range: -20 to 40°C). Dash line represent room temperature (RT).

Selected samples were investigated using transmission electron microscope (TEM) (Fig. 7 and Fig. 8). Micrographs were collected on TECNAI G2 F20 (200 kV) coupled with an energy dispersive X-ray spectrometer (EDAX).

TEM micrographs reveal well dispersed lithium manganese orthosilicate grains (dark grey and black dots) in the carbon matrix. Figures 7a; 7c; 7d present C/Li₂MnSiO₄@600 at different magnifications. Fig. 7b shows STEM-HAADF (Scanning Transmission Electron Microscopy – High Angular Annular Dark Field) image of the C/Li₂MnSiO₄@600 composite. Energy dispersive X-ray spectroscopy (EDS) (Fig. 7b1; 7b2) confirmed presence of lithium manganese silicate in the spots marked in the micrograph 7b (Cu peaks in all EDS spectra originate from TEM copper grid). High resolution electron microscopy image (HREM) shows single grain of Li₂MnSiO₄ (Fig. 7e). Corresponding to that IFFT (Inverse Fast Furrier Transform) image (Fig. 7f) clearly shows a crystalline structure of obtained material which can be identified as Li₂MnSiO₄. Grains in the composite have sizes in a range of 5-10 nm. Microstructure of a C/Li₂MnSiO₄ composite calcined at 700 °C is shown in Fig. 8. C/Li₂MnSiO₄@700 °C composite has a bimodal distribution of a crystallite size. TEM micrographs show presence of small (5-10 nm) Li₂MnSiO₄ crystallites, analogues of C/Li₂MnSiO₄@600 °C and a bigger, well crystallized silicates grains, in a range of 50-150 nm (Fig. 8a, c, d). Both types of crystallites in the sample have the same composition which is confirmed by EDS analysis presented in fig. 8b₁, b₂, b₃.

Figure 7.

TEM micrographs of C/Li₂MnSiO₄@600. Micrographs a), c) and d) present bright field images of C/Li₂MnSiO₄@600 composite in different magnification; b) microstructure image observed in STEM-HAADF with EDS analysis (b₁; b₂) from marked points; e) high resolution micrograph (HREM) of a single grain and IFFT image f) from the same region.

Figure 8.

TEM micrographs of C/Li₂MnSiO₄@700. Micrographs a), c) and d) presents bright field images of C/Li₂MnSiO₄@700 composite in different magnification; b) microstructure image observed in STEM-HAADF with EDS analysis (b₁; b₂; b₃) from selected points; d₁) high resolution micrograph (HREM) of region marked in Fig. 8d; d₂) IFFT image from square marked in d₁).

STEM-HAADF image (Fig. 8b) from the same region as a bright field Fig. 8a shows points from where EDS analysis was conducted. All of lithium manganese orthosilicate crystallites in C/Li₂MnSiO₄@700 °C are well dispersed and fully covered in amorphous carbon. HREM image in Fig. 8d₁ with IFFT in Fig. 8d₂ shows crystalline structure of Li₂MnSiO₄ grain.

5. C/Li₂MnSiO₄ composite cathode material

Nevertheless obtained composites show homogeneous distribution of particles in carbon matrix and exhibit good electrical conductivity they work poorly in a battery cell (see paragraph 5.2). Thickness of primary carbon coating on active material grains strongly limits the electrochemical performance of composite.

5.1. CCL/Li₂MnSiO₄ composite preparation and properties

Preparation of composites with well define morphology of carbon layers and optimal carbon content was achieved by burning out the primary carbon and recoating Li₂MnSiO₄ nanosized grains with conductive carbon layers (CCL/Li₂MnSiO₄ composites). Burning out of primary carbon formed in the sample during synthesis process was carried through calcination of C/Li₂MnSiO₄@600 and C/Li₂MnSiO₄@700 under air flow at 300 °C for 3 h. CCL/Li₂MnSiO₄ composites were produced by wet polymer precursor deposition on active material grains and subsequent controlled pyrolysis [14, 20, 34]. Poly-N-vinylformamide (PNVF) obtained by radical-free polymerization of N-vinylformamide (Aldrich) with pyromellitic acid (PMA) additive (5-10 wt%) was used as a carbon polymer precursor [34]. To achieve an impregnation, Li₂MnSiO₄ grains were suspended in water polymer solution (8-15 wt%). Finally, samples were dried up in an air drier at 90 °C for 24 h. Prepared samples were pyrolyzed at 600 °C for 6 h under inert atmosphere (Ar).

Burning out primary carbon from the surface of lithium manganese silicate in air atmosphere leads to partial decomposition of active material. Fig. 9 shows XRD patterns of C/Li₂MnSiO₄@600, Li₂MnSiO₄@600 without carbon and CCL/Li₂MnSiO₄@600 composite after coating with CCL from polymer precursor.

In diffraction patterns of Li₂MnSiO₄@600 it is clearly visible that after burning out primary carbon layer new phase appears in the sample (LiMn₂O₄). Proposed carbon coating process can reverse decomposition of Li₂MnSiO₄. After coating Li₂MnSiO₄ (Pmn2₁) is a dominant phase in CCL/Li₂MnSiO₄@600 and all the lines, except for very low intensity line around 19°, can be associated with Li₂MnSiO₄ phase. CCL preparation procedure influence active material grain size. Average crystallite size calculated using Scherer equation in CCL/Li₂MnSiO₄@600 is ca. 25 nm. TEM analysis confirms increase of crystallites diameters. Li₂MnSiO₄ grains observed under TEM are in the range 25-75 nm. TEM micrographs of CCL/Li₂MnSiO₄@600 are presented in Fig. 10.

Figure 9.

XRD patterns of C/Li₂MnSiO₄@600, Li₂MnSiO₄@600 and CCL/Li₂MnSiO₄@600.

Bright field micrographs (Fig. 10a, c, d) shows crystallites of Li₂MnSiO₄ covered with conductive carbon layers (CCL). Formed carbon coatings adhere well to the surface of active material grains, no voids are visible at the CCL/Li₂MnSiO₄ interface (Fig. 10e). Silicate grains are uniformly covered with approximately 4-5 nm thick CCL. EDS analysis once again confirm presence of lithium manganese silicate in the composite (Fig. 10b, b₁). Carbon content in samples was calculated from TPO measurements and they are displayed in Table 2.

Sample name	Precursor calcination temperature	Carbon content
CCL/Li2MnSiO4@600_7.5%	600 °C	7.5%
CCL/Li2MnSiO4@600_12%	600 °C	12%
CCL/Li2MnSiO4@700_5%	700 °C	5%
CCL/Li2MnSiO4@700_10%	700 °C	10%

Table 2.

Carbon content of CCL/Li₂MnSiO₄ composites.

Figure 10.

TEM micrographs of CCL/Li₂MnSiO₄@600. Micrographs a), c) and d) presents bright field images of CCL/Li₂MnSiO₄@600 composite; b) microstructure image observed in STEM-HAADF with EDS analysis (b₁) from selected point; e) HREM micrograph of carbon coating on the active material grain surface.

5.2. Electrochemical properties of C/Li₂MnSiO₄ composite cathode

The charge-discharge cycling studies of Li/Li⁺/(C/Li₂MnSiO₄) cells were conducted in a four electrode configuration using CR2032 assembly between 2.7 and 4.7 V at C/200 rate at room temperature. LiPF₆ solution 1M in EC/DEC (1:1) was used in the cells as an electrolyte. The galvanostatic measurements were carried out using ATLAS 0961 MBI test system. Charge-discharge tests were conducted for all CCL composites listed in table 2, composites with primary carbon (C/Li₂MnSiO₄@600, C/Li₂MnSiO₄@700 and C/Li₂MnSiO₄@800) and standard composites (Li₂MnSiO₄@600_CB and Li₂MnSiO₄@700_CB). Standard composites were prepared by mixing Li₂MnSiO₄ powder with commercial carbon additive – carbon black (CB). 15 wt.% of carbon was used. Results collected from charge/discharge tests are presented in fig. 11-16.

C/Li₂MnSiO₄@600 sample did not show any reversible capacity (Fig. 11a). Lack of electrochemical activity of C/Li₂MnSiO₄@600 is connected with too high carbon loading. High carbon content (34%) is responsible for limiting of ionic conductivity in the composite and for surface polarization. Galvanostatic cycling studies of C/Li₂MnSiO₄@700 and C/Li₂MnSiO₄@800 revealed reversible capacity in a range of 30 mAh g^-1. Fig. 11b show charge and discharge capacity for 7 cycles of C/Li₂MnSiO₄@700 composite. After first four cycles reversible capacity stabilizes at about 30 mAh g^-1. Very poor performance of this sample is also connected with high amount of carbon in prepared composite (32%).

Standard composites obtained by mixing active silicate materials with carbon black (15%) show extremely low reversible capacity as well (Fig. 12a and 12b). In this case, carbon additive does not provide sufficient electrical contact between active material grains. Due to this fact, even under low C/200 rate samples performed very poorly in charge/discharge tests.

Figure 11.

Cell cycling behavior of: a) C/Li₂MnSiO₄@600, b) C/Li₂MnSiO₄@700.

Figure 12.

Cell cycling behavior of standard composites Li2MnSiO4 + carbon black (15% carbon content)

Figure 13.

Cell cycling behavior of CCL/Li₂MnSiO₄@600 (7.5% carbon content)

Figure 14.

Cell cycling behavior of CCL/Li₂MnSiO₄@600 (12% carbon content)

Figure 15.

Cell cycling behavior of CCL/Li₂MnSiO₄@700 (5% carbon content)

Figure 16.

Cell cycling behavior of CCL/Li₂MnSiO₄@700 (10% carbon content)

Fig. 13-16 show cycling behavior of CCL composites with different amounts of carbon. Delithation of CCL composites after initial charging is in the range of 48-68% (charged CCL/Li₂MnSiO₄@600_12%C – correspond to composition Li_1.04MnSiO₄ and charged CCL/Li₂MnSiO₄@700_10%C – correspond to composition Li_0.64MnSiO₄). The observed capacity loss in the first cycles is related to SEI formation in CCL. During lithium extraction, Mn³⁺ and Mn⁴⁺ appear, and at the same time Li₂MnSiO₄ undergoes decomposition caused by Jahn-Teller distortion associated with changes in lattice parameters during Mn³⁺ → Mn⁴⁺ transition [30]. Despite the fact that crystalline structure collapses, discharge capacity is around 90-110 mAh/g. CCL composites produced from Li₂MnSiO₄ calcined at 600 °C exhibit better coulombic efficiency than Li₂MnSiO₄ calcined at 700 °C, the reversible capacity after 10 cycle is close to 100 mAh/g. The results indicate that coulombic efficiency of cathode material depends on grain size and homogeneity in grain size distribution, while cell capacity is limited by carbon coating performance.

6. Conclusions

Dilithium manganese orthosilicate – a high energy density cathode material – was successfully synthesized by sol-gel Pechini method. Encapsulation of nanosized grains of Li₂MnSiO₄ in carbon matrix, resulted from organic precursor, avoided further sintering. Different crystallite size was obtained, in particular, nanoparticles within range of 5-10 nm. High chemical stability of this material under a highly reductive environment was observed. Application of carbon coating improved electrical conductivity of cathode material to the satisfactory level of ~10^-0.5 S/cm. Thickness of carbon coating on active material grains strongly limits the electrochemical performance of composite. Formation of CCL/Li₂MnSiO₄ composites significantly improved electrochemical performance of cathode materials, showing a reversible capacity of 90-110 mAh/g after 10 cycles. Electrochemical tests indicated that composite preparation should be optimized in terms of carbon content, CCL performance and homogeneity of a crystallite size. It was found that coulombic efficiency of cathode material depends on grain size and homogeneity in grain size distribution, while cell capacity is limited by carbon coating performance.

Acknowledgement

This work has been financially supported by the Polish National Science Centre under research grant no. N N209 088638, and by the European Institute of Innovation and Technology under the KIC InnoEnergy NewMat project. The part of the measurements was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (POIG.02.01.00-12-023/08). One of the authors (M.Ś.) acknowledge a financial support from the International PhD-studies programme at the Faculty of Chemistry Jagiellonian University within the Foundation for Polish Science MPD. TEM analysis were carried out in Laboratory of Transmission Analytical Electron Microscopy at the Institute of Metallurgy and Material Science, Polish Academy of Sciences.