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Synthesis and characterization of LiNi0.85Co0.15−2x(TiMg)xO2 as cathode materials for lithium-ion batteries
Materials Chemistry and Physics 88 (2004) 145–149
Synthesis and characterization of LiNi0.85Co0.15−2x (TiMg)x O2 as cathode materials for lithium-ion batteries Xianjun Zhu, Honghao Chen, Hui Zhan, Dailing Yang, Yunhong Zhou∗ Department of chemistry, Wuhan University, Wuhan 430072, PR China Received 11 January 2004; received in revised form 28 June 2004; accepted 29 June 2004
Abstract Samples of LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05), prepared by solid state reaction at 725 ◦ C for 24 h from LiOH·H2 O, Ni2 O3 , Co2 O3 , TiO2 and Mg(OH)2 under oxygen flow, were characterized by TG–DTA, XRD and electrochemical tests. LiNiO2 simultaneously doped by Co-Ti-Mg has been tried to improve the cathode performance. The results show that co-doping (x = 0.025) definitely has a large beneficial effect in increasing the capacity (182.7 mA h g−1 of the first discharge capacity for LiNi0.85 Co0.10 (TiMg)0.025 O2 ) and cycling behavior (170.6 mA h g−1 after 15 cycles). Differential capacity versus voltage curves indicate the co-doped LiNiO2 show suppression of the phase transitions as compared with LiNiO2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Cathode materials; LiNi0.85 Co0.15−2x (TiMg)x O2 ; Co-doping
1. Introduction Lithium-ion batteries have received great attention in recent years due to their application in portable electronic devices as well as potential long-term candidates for a zeroemission electric vehicle. The common positive material in commercial lithium-ion batteries is LiCoO2 [1], but environmental constrains have created pressures to replace lithium cobaltate with other materials [2–5]. Among these cathode materials, LiNiO2 has been considered as one of the most promising cathode materials for lithium-ion batteries because of its high specific capacity, low cost and low toxicity. However, its major disadvantages are difficulty in synthesizing stoichiometric phase, capacity fading with cycling resulted from structure instability, and poor thermal stability [6–8]. Solid solutions involving both LiCoO2 and LiNiO2 , layered cathode materials of the general formula LiNi1−x Cox O2 (0 < x < 1) have been scrutinized for possible commercial applications [9–12]. These solid solutions enjoy the advantages of both the end members with better cyclability and ∗
Corresponding author. Tel.: +86 27 87686931; fax: +86 27 87647617. E-mail addresses: [email protected], [email protected] (Y. Zhou).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.06.042
safety aspects, because the presence of cobalt stabilizes the structure in a strictly two-dimensional lattice. Nevertheless, overcharge problems with these materials are still unresolved [13,14]. It is now accepted that thermal and chemical instability of the fully delithiated oxides is a great part responsibility for cycling failure. In an attempt to prevent material degradation during more extraction of lithium, multiple element substitution brings some peculiar advantages on reversibility, capacity fading or thermal stability for safety, and a unique combination of two or more cation substitution for Ni3+ in LiNiO2 has some promising features [15–17]. In order to develop nickel-based lithium transition metal oxides with a high reversible capacity, in this paper, co-doped compounds LiNi0.85 Co0.15−2x (TiMg)x O2 were synthesized. Results show an improved and sustained capacity (on cycling) and a large suppression of the phase transitions compared with the pristine LiNiO2 .
2. Experimental All starting materials were reagent grade. Samples LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05)
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were synthesized by a solid state reaction. For instance, the LiNi0.85 Co0.10 (TiMg)0.025 O2 was prepared by mixing LiOH·H2 O, Ni2 O3 , Co2 O3 , TiO2 and Mg(OH)2 in a mole ratio of Li/Ni/Co/Ti/Mg = 1.07:0.85:0.10:0.025:0.025. Excess lithium compensates for lithium vaporization at high temperature. The mixture was preheated at 600 ◦ C for 6 h under a stream of oxygen, and then ground and pelletized, followed by heating under oxygen flow in a tubular furnace (SR100-500/11, GERO Hochtemperatur¨ofen GmbH D75242 Neuhausen) at 725 ◦ C for 24 h. The thermal and weight changes in the synthetic process were investigated with the simultaneous recording of weight loss (TG) and temperature variation (DTA). TG and DTA measurements of the precursor material were performed between room temperature and 1000 ◦ C at 10 ◦ C min−1 under flowing oxygen (60 ml min−1 ) by means of a WCT-1A type thermal analyzer (Beijing Optical Instrument Factory, China). Powder X-ray diffraction (XRD) was used to identify the crystalline phase of the materials (Rigaku, D/max-RB, CuK␣ radiation). The electrochemical performance of the materials was studied at room temperature by assembling 2016-type coin cells with a lithium metal anode. The cathode used had a composition of 80 wt.% active materials, 15 wt.% acetylene black (as conducting agent) and 5 wt.% PTFE (ploytetrafluorethylene) binder. The electrolyte was 1 M LiPF6 in 1:1 (weight ratio) EC/DEC (Merck, Germany). The Celgard 2300 membrance was used as cell separator. All manipulations were performed in a glove box filled with Ar under an oxygen level less than 5 ppm and water content less than 1 ppm. The galvanostatic charge–discharge experiments were undertaken at the current of 18 mA g−1 between 3.0 and 4.3 V in a multichannel battery cycling unit (Arbin BT2000, USA).
3. Results and discussion
Fig. 1. TG–DTA curves of the raw material mixture used for synthesizing LiNi0.85 Co0.10 (TiMg)0.025 O2 .
endothermic peaks at 424 and 474 ◦ C in DTA curve, respectively. Between 600 and 750 ◦ C, TG curve is almost flat. It is reasonable to be regarded as the formation of the layered crystalline LiNi0.85 Co0.15−2x (TiMg)x O2 . In the temperature range of higher than 750 ◦ C, there is a continuous weight loss process which is attributed to part decomposition of the as-formed crystalline, resulting in non-stoichiometric compounds or non-layered crystalline. It can be seen that there was endothermic phenomenon in DTA curve. Although the layered lattice began to be formed at around 600 ◦ C, the mixture must be calcined for the complete layered crystallization at 725 ◦ C for 24 h, as evidenced by the X-ray diffraction studies. 3.2. Structural studies The X-ray diffraction patterns of the LiNi0.85 Co0.15−2x (TiMg)x O2 and LiNiO2 are presented in Fig. 2.
3.1. Thermal studies In order to gain an understanding of the thermal and weight change as well as the crystalline transformation during the synthesizing process, and to determine preheating and calcining temperature of the precursor, TG and DTA were performed. The TG and DTA curves are presented in Fig. 1. The thermogravigram (TG) shows that weight loss takes place in several steps. The first step which occurred between room temperature and 150 ◦ C is due to the removal of water from LiOH·H2 O, which is associated with the sharp endothermic peak at 98 ◦ C in DTA curve. The second step of 1% weight loss or so in the temperature interval of 300 and 400 ◦ C is attributed to the decomposition of Mg(OH)2 , corresponding to a slight endothermic peak at 330 ◦ C in DTA curve. The third step of 2% weight loss occurred between 400 and 600 ◦ C. During this course, LiOH melted and subsequently decomposed, and it is evidenced by two middle sharp
Fig. 2. XRD patterns of LiNi0.85 Co0.15−2x (TiMg)x O2 . (a) 2θ: 10.0–80.0◦ , (b) 2θ: 37.5–39.0◦ , (c) 2θ: 62.0–67.0◦ .
X. Zhu et al. / Materials Chemistry and Physics 88 (2004) 145–149
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Fig. 4. The first charge–discharge curves.
Fig. 3. Plot of cell parameters as a function of x in LiNi0.85 Co0.15−2x (TiMg)x O2 (䊉) and LiNiO2 ().
No impurity phase was detected by XRD analysis. This indicates that Co3+ , Ti4+ and Mg2+ ions are compatible in the layered R3m hexagonal structure and pure-phase solid was obtained. Compounds with higher x values were found to contain impurity phase and hence were not pursed further. With the increase of Ti and Mg co-doping content in the structure, the (0 0 3) peak was observed to decrease and broaden, and the (0 0 3)–(1 0 4) peak intensity ratio I0 0 3 /I1 0 4 also decreases, as compared with those of LiNiO2 [18]. This suggests that there exists a microscopic structure change, which is probably caused by the mismatch of Co3+ , Ti4+ and Mg2+ with Ni3+ due to the difference in their radii. In such structures, the oxygen sublattice can be considered as a close-packed centered cubic (FCC) lattice with a distortion in the c direction (or in (1 1 1) direction if a cubic index system is used). Because of the slight distortion in the c direction, the value of the c/a of LiNiO2 is 4.934, leading to a splitting of the 0 0 6–0 1 2 and 0 1 8–1 1 0 peak pairs. When the distortion in the c direction is absent (or the structure is totally cubic), the ratio of the lattice constant c/a is 4.899, and the 0 0 6–0 1 2 or 0 1 8–1 1 0 peak pair in the diffraction pattern merges into a single peak. With the addition of Ti4+ and Mg2+ , however, these peaks gradually broaden and split more clearly. All the diffraction peaks shift to lower angle with an increase of Ti4+ and Mg2+ content. The lattice constants were calculated by using a least-square method. Fig. 3 shows the lattice parameters a, c, c/a and the unit cell volume of the Co-Ti-Mg samples as a function of x in LiNi0.85 Co0.15−2x (TiMg)x O2 . Both a and c increase with an increase in the Ti and Mg co-doping level, the hexagonal unit cell volume also increases correspondingly. This means that the lattice expands with the introduction of Ti and Mg. On the ˚ low spin) has smaller ionic radius one hand, Co3+ (0.53 A, 3+ ˚ than Ni (0.56 A, low spin), leading to decrease in the unit
˚ c = 14.166 A ˚ cell volume of LiNi0.85 Co0.15 O2 (a = 2.870 A, 3 ˚ and V = 101.06 A ) compared with that of LiNiO2 (a = ˚ c = 14.203 A ˚ and V = 101.95 A ˚ 3 ). On the other hand, 2.879 A, 4+ 2+ ˚ respectively) is the radii of Ti and Mg (0.61 and 0.72 A, 3+ larger than that of Ni , resulting in the increase of the unit cell volume of the sample LiNi0.85 Co0.15−2x (TiMg)x O2 with Ti and Mg content compared with that of LiNi0.85 Co0.15 O2 . Because Ti4+ and Mg2+ ions seem to have different local ionic ordering from the Ni3+ ions in the lattice due to different valence, the lattice of the co-doped samples tends to form more ionic (Ni0.85 Co0.15−2x (TiMg)x )n sheets, leading to have relatively higher c/a values than does LiNiO2 . Interestingly, the c/a ratio increases from x = 0.00 to x = 0.025, and decreases at x = 0.05. This suggests that, when the x value is bigger, the layered characteristics of the lattice decreases, or have more cubic one, which is evidenced by the decreased height of the (0 0 3) peak of LiNi0.85 Co0.05 (TiMg)0.05 O2 in the XRD pattern. 3.3. Electrochemical studies Fig. 4 shows the first charge–discharge curves for LiNi0.85 Co0.10 (TiMg)0.025 O2 and LiNiO2 at a constant current density of 18 mA g−1 in the voltage range of 3.0–4.3 V. The first charge and discharge capacities of the doped sample are 221.7 and 182.7 mA h g−1 , respectively, and those of LiNiO2 are 218.0 and 180.7 mA h g−1 , respectively. The capacity loss at the first cycle in both is equal to about 0.14 of lithium. These observations are similar to those reported in the literature [19]. The main reason for the capacity loss is due to the formation of electrochemically inactive domain in the cathode materials [20]. Additionally, for the doped sample, there is the displacement of the dopant Ti4+ and Mg2+ ions to the interslab space, leading to the occupancy of the available sites by reintercalation of lithium ions [21]. As shown in Fig. 4, the charge voltage of the 2.5% Ti + Mg doped sample is somewhat lower and its discharge voltage is some-
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Fig. 5. Cyclability of LiNi0.85 Co0.15−2x (TiMg)x O2 . 1: x = 0.0125, 2: x = 0.025, 3: x = 0.05, 4: LiNiO2 .
what higher than those of LiNiO2 . Further, the charge and discharge curves of the sample with 2.5% Ti + Mg are flater than those of LiNiO2 . The cyclability of the Co-Ti-Mg co-doped samples and pure LiNiO2 are shown in Fig. 5. The first discharge capacity decreases with an increase of Ti and Mg content. As known, cobalt, which is similar to nickel, is electrochemically active, and it can stabilize the structure in a two-dimension lattice. Titanium and magnesium, which act as “pillar” in the interslab space, are not electrochemically active when substituted at the Ni-site in the co-doped samples. Thus, they are expected to result in the decrease in capacity, depending on the dopant concentration. Although the 5% Ti + Mg sample shows better cyclability (94.3% up to 15 cycles) compared to 83.4, 87.5 and 93.4% for LiNiO2 , 1.25 and 2.5% Ti + Mg samples, respectively, its discharge capacity is lower. Compromising high specific capacity and good cyclability, the results suggest that the optimal dopant of Ti and Mg for LiNi0.85 Co0.15−2x (TiMg)x O2 is about x = 0.025. Fig. 6 shows the derivative dQ/dV (the first charge–discharge cycle) for the co-doped samples with different x. As shown, pure LiNiO2 characteristically exhibits many sharp peaks during the course of lithium intercalation-deintercalation, which have been thoroughly studied by in situ XRD and derivative studies on active electrochemical cells [19,20,22]. The sharp peaks are indicative of first-order phase transitions and two-phase co-existence and the broad ones are one-phase continuous phase transitions. During the charging process, the hexagonal phase changes from H1 to monoclinic (M) and then from M to H2 and finally H2 to H3 where H1, H2 and H3 represent phases with hexagonal symmetry but changed lattice parameters. During the discharging course, these phase transitions are reversible with a slight hysteresis. For Co-Ti-Mg co-doped samples, the shape of the derivative curves changes drastically, and the intensity of the peaks decreases with an increase of Ti and Mg doping content.
Fig. 6. The derivative dQ/dV of the first charge–discharge cycle for LiNi0.85 Co0.15−2x (TiMg)x O2 and LiNiO2 . H1, M1, H2 and H3 refer to different crystallographic phases.
This is suggestive that these transitions are suppressed. Thus, the change in derivative curves combined with XRD above, demonstrates that Co, Ti and Mg all are indeed incorporated into the structure, and this incorporation causes beneficial changes in the electrochemical cycling of lithium in the cathode material.
4. Conclusions Co-Ti-Mg co-doped samples of LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05) were synthesized by solid state reaction. Electrochemical studies showed that the co-doped samples have large beneficial effect on cycling reversibility compared with the pristine LiNiO2 . LiNi0.85 Co0.10 (TiMg)0.025 O2 give 182.7 mA h g−1 of the first discharge capacity, and the capacity can be largely remained after 15 cycles (retention ratio: 93.4%). Co, Ti and Mg (molar ratio Ti/Mg = 1.0) doped in LiNiO2 structure can suppress crystallographic phase transitions and decrease the irreversible capacity. Compromising high specific capacity and good cyclability, the LiNi0.85 Co0.10 (TiMg)0.025 O2 may be a good candidate cathode material for Li-ion batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 29833090).
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Synthesis and characterization of LiNi0.85Co0.15−2x (TiMg)x O2 as cathode materials for lithium-ion batteries Xianjun Zhu, Honghao Chen, Hui Zhan, Dailing Yang, Yunhong Zhou∗ Department of chemistry, Wuhan University, Wuhan 430072, PR China Received 11 January 2004; received in revised form 28 June 2004; accepted 29 June 2004
Abstract Samples of LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05), prepared by solid state reaction at 725 ◦ C for 24 h from LiOH·H2 O, Ni2 O3 , Co2 O3 , TiO2 and Mg(OH)2 under oxygen flow, were characterized by TG–DTA, XRD and electrochemical tests. LiNiO2 simultaneously doped by Co-Ti-Mg has been tried to improve the cathode performance. The results show that co-doping (x = 0.025) definitely has a large beneficial effect in increasing the capacity (182.7 mA h g−1 of the first discharge capacity for LiNi0.85 Co0.10 (TiMg)0.025 O2 ) and cycling behavior (170.6 mA h g−1 after 15 cycles). Differential capacity versus voltage curves indicate the co-doped LiNiO2 show suppression of the phase transitions as compared with LiNiO2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Cathode materials; LiNi0.85 Co0.15−2x (TiMg)x O2 ; Co-doping
1. Introduction Lithium-ion batteries have received great attention in recent years due to their application in portable electronic devices as well as potential long-term candidates for a zeroemission electric vehicle. The common positive material in commercial lithium-ion batteries is LiCoO2 [1], but environmental constrains have created pressures to replace lithium cobaltate with other materials [2–5]. Among these cathode materials, LiNiO2 has been considered as one of the most promising cathode materials for lithium-ion batteries because of its high specific capacity, low cost and low toxicity. However, its major disadvantages are difficulty in synthesizing stoichiometric phase, capacity fading with cycling resulted from structure instability, and poor thermal stability [6–8]. Solid solutions involving both LiCoO2 and LiNiO2 , layered cathode materials of the general formula LiNi1−x Cox O2 (0 < x < 1) have been scrutinized for possible commercial applications [9–12]. These solid solutions enjoy the advantages of both the end members with better cyclability and ∗
Corresponding author. Tel.: +86 27 87686931; fax: +86 27 87647617. E-mail addresses: [email protected], [email protected] (Y. Zhou).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.06.042
safety aspects, because the presence of cobalt stabilizes the structure in a strictly two-dimensional lattice. Nevertheless, overcharge problems with these materials are still unresolved [13,14]. It is now accepted that thermal and chemical instability of the fully delithiated oxides is a great part responsibility for cycling failure. In an attempt to prevent material degradation during more extraction of lithium, multiple element substitution brings some peculiar advantages on reversibility, capacity fading or thermal stability for safety, and a unique combination of two or more cation substitution for Ni3+ in LiNiO2 has some promising features [15–17]. In order to develop nickel-based lithium transition metal oxides with a high reversible capacity, in this paper, co-doped compounds LiNi0.85 Co0.15−2x (TiMg)x O2 were synthesized. Results show an improved and sustained capacity (on cycling) and a large suppression of the phase transitions compared with the pristine LiNiO2 .
2. Experimental All starting materials were reagent grade. Samples LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05)
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were synthesized by a solid state reaction. For instance, the LiNi0.85 Co0.10 (TiMg)0.025 O2 was prepared by mixing LiOH·H2 O, Ni2 O3 , Co2 O3 , TiO2 and Mg(OH)2 in a mole ratio of Li/Ni/Co/Ti/Mg = 1.07:0.85:0.10:0.025:0.025. Excess lithium compensates for lithium vaporization at high temperature. The mixture was preheated at 600 ◦ C for 6 h under a stream of oxygen, and then ground and pelletized, followed by heating under oxygen flow in a tubular furnace (SR100-500/11, GERO Hochtemperatur¨ofen GmbH D75242 Neuhausen) at 725 ◦ C for 24 h. The thermal and weight changes in the synthetic process were investigated with the simultaneous recording of weight loss (TG) and temperature variation (DTA). TG and DTA measurements of the precursor material were performed between room temperature and 1000 ◦ C at 10 ◦ C min−1 under flowing oxygen (60 ml min−1 ) by means of a WCT-1A type thermal analyzer (Beijing Optical Instrument Factory, China). Powder X-ray diffraction (XRD) was used to identify the crystalline phase of the materials (Rigaku, D/max-RB, CuK␣ radiation). The electrochemical performance of the materials was studied at room temperature by assembling 2016-type coin cells with a lithium metal anode. The cathode used had a composition of 80 wt.% active materials, 15 wt.% acetylene black (as conducting agent) and 5 wt.% PTFE (ploytetrafluorethylene) binder. The electrolyte was 1 M LiPF6 in 1:1 (weight ratio) EC/DEC (Merck, Germany). The Celgard 2300 membrance was used as cell separator. All manipulations were performed in a glove box filled with Ar under an oxygen level less than 5 ppm and water content less than 1 ppm. The galvanostatic charge–discharge experiments were undertaken at the current of 18 mA g−1 between 3.0 and 4.3 V in a multichannel battery cycling unit (Arbin BT2000, USA).
3. Results and discussion
Fig. 1. TG–DTA curves of the raw material mixture used for synthesizing LiNi0.85 Co0.10 (TiMg)0.025 O2 .
endothermic peaks at 424 and 474 ◦ C in DTA curve, respectively. Between 600 and 750 ◦ C, TG curve is almost flat. It is reasonable to be regarded as the formation of the layered crystalline LiNi0.85 Co0.15−2x (TiMg)x O2 . In the temperature range of higher than 750 ◦ C, there is a continuous weight loss process which is attributed to part decomposition of the as-formed crystalline, resulting in non-stoichiometric compounds or non-layered crystalline. It can be seen that there was endothermic phenomenon in DTA curve. Although the layered lattice began to be formed at around 600 ◦ C, the mixture must be calcined for the complete layered crystallization at 725 ◦ C for 24 h, as evidenced by the X-ray diffraction studies. 3.2. Structural studies The X-ray diffraction patterns of the LiNi0.85 Co0.15−2x (TiMg)x O2 and LiNiO2 are presented in Fig. 2.
3.1. Thermal studies In order to gain an understanding of the thermal and weight change as well as the crystalline transformation during the synthesizing process, and to determine preheating and calcining temperature of the precursor, TG and DTA were performed. The TG and DTA curves are presented in Fig. 1. The thermogravigram (TG) shows that weight loss takes place in several steps. The first step which occurred between room temperature and 150 ◦ C is due to the removal of water from LiOH·H2 O, which is associated with the sharp endothermic peak at 98 ◦ C in DTA curve. The second step of 1% weight loss or so in the temperature interval of 300 and 400 ◦ C is attributed to the decomposition of Mg(OH)2 , corresponding to a slight endothermic peak at 330 ◦ C in DTA curve. The third step of 2% weight loss occurred between 400 and 600 ◦ C. During this course, LiOH melted and subsequently decomposed, and it is evidenced by two middle sharp
Fig. 2. XRD patterns of LiNi0.85 Co0.15−2x (TiMg)x O2 . (a) 2θ: 10.0–80.0◦ , (b) 2θ: 37.5–39.0◦ , (c) 2θ: 62.0–67.0◦ .
X. Zhu et al. / Materials Chemistry and Physics 88 (2004) 145–149
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Fig. 4. The first charge–discharge curves.
Fig. 3. Plot of cell parameters as a function of x in LiNi0.85 Co0.15−2x (TiMg)x O2 (䊉) and LiNiO2 ().
No impurity phase was detected by XRD analysis. This indicates that Co3+ , Ti4+ and Mg2+ ions are compatible in the layered R3m hexagonal structure and pure-phase solid was obtained. Compounds with higher x values were found to contain impurity phase and hence were not pursed further. With the increase of Ti and Mg co-doping content in the structure, the (0 0 3) peak was observed to decrease and broaden, and the (0 0 3)–(1 0 4) peak intensity ratio I0 0 3 /I1 0 4 also decreases, as compared with those of LiNiO2 [18]. This suggests that there exists a microscopic structure change, which is probably caused by the mismatch of Co3+ , Ti4+ and Mg2+ with Ni3+ due to the difference in their radii. In such structures, the oxygen sublattice can be considered as a close-packed centered cubic (FCC) lattice with a distortion in the c direction (or in (1 1 1) direction if a cubic index system is used). Because of the slight distortion in the c direction, the value of the c/a of LiNiO2 is 4.934, leading to a splitting of the 0 0 6–0 1 2 and 0 1 8–1 1 0 peak pairs. When the distortion in the c direction is absent (or the structure is totally cubic), the ratio of the lattice constant c/a is 4.899, and the 0 0 6–0 1 2 or 0 1 8–1 1 0 peak pair in the diffraction pattern merges into a single peak. With the addition of Ti4+ and Mg2+ , however, these peaks gradually broaden and split more clearly. All the diffraction peaks shift to lower angle with an increase of Ti4+ and Mg2+ content. The lattice constants were calculated by using a least-square method. Fig. 3 shows the lattice parameters a, c, c/a and the unit cell volume of the Co-Ti-Mg samples as a function of x in LiNi0.85 Co0.15−2x (TiMg)x O2 . Both a and c increase with an increase in the Ti and Mg co-doping level, the hexagonal unit cell volume also increases correspondingly. This means that the lattice expands with the introduction of Ti and Mg. On the ˚ low spin) has smaller ionic radius one hand, Co3+ (0.53 A, 3+ ˚ than Ni (0.56 A, low spin), leading to decrease in the unit
˚ c = 14.166 A ˚ cell volume of LiNi0.85 Co0.15 O2 (a = 2.870 A, 3 ˚ and V = 101.06 A ) compared with that of LiNiO2 (a = ˚ c = 14.203 A ˚ and V = 101.95 A ˚ 3 ). On the other hand, 2.879 A, 4+ 2+ ˚ respectively) is the radii of Ti and Mg (0.61 and 0.72 A, 3+ larger than that of Ni , resulting in the increase of the unit cell volume of the sample LiNi0.85 Co0.15−2x (TiMg)x O2 with Ti and Mg content compared with that of LiNi0.85 Co0.15 O2 . Because Ti4+ and Mg2+ ions seem to have different local ionic ordering from the Ni3+ ions in the lattice due to different valence, the lattice of the co-doped samples tends to form more ionic (Ni0.85 Co0.15−2x (TiMg)x )n sheets, leading to have relatively higher c/a values than does LiNiO2 . Interestingly, the c/a ratio increases from x = 0.00 to x = 0.025, and decreases at x = 0.05. This suggests that, when the x value is bigger, the layered characteristics of the lattice decreases, or have more cubic one, which is evidenced by the decreased height of the (0 0 3) peak of LiNi0.85 Co0.05 (TiMg)0.05 O2 in the XRD pattern. 3.3. Electrochemical studies Fig. 4 shows the first charge–discharge curves for LiNi0.85 Co0.10 (TiMg)0.025 O2 and LiNiO2 at a constant current density of 18 mA g−1 in the voltage range of 3.0–4.3 V. The first charge and discharge capacities of the doped sample are 221.7 and 182.7 mA h g−1 , respectively, and those of LiNiO2 are 218.0 and 180.7 mA h g−1 , respectively. The capacity loss at the first cycle in both is equal to about 0.14 of lithium. These observations are similar to those reported in the literature [19]. The main reason for the capacity loss is due to the formation of electrochemically inactive domain in the cathode materials [20]. Additionally, for the doped sample, there is the displacement of the dopant Ti4+ and Mg2+ ions to the interslab space, leading to the occupancy of the available sites by reintercalation of lithium ions [21]. As shown in Fig. 4, the charge voltage of the 2.5% Ti + Mg doped sample is somewhat lower and its discharge voltage is some-
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Fig. 5. Cyclability of LiNi0.85 Co0.15−2x (TiMg)x O2 . 1: x = 0.0125, 2: x = 0.025, 3: x = 0.05, 4: LiNiO2 .
what higher than those of LiNiO2 . Further, the charge and discharge curves of the sample with 2.5% Ti + Mg are flater than those of LiNiO2 . The cyclability of the Co-Ti-Mg co-doped samples and pure LiNiO2 are shown in Fig. 5. The first discharge capacity decreases with an increase of Ti and Mg content. As known, cobalt, which is similar to nickel, is electrochemically active, and it can stabilize the structure in a two-dimension lattice. Titanium and magnesium, which act as “pillar” in the interslab space, are not electrochemically active when substituted at the Ni-site in the co-doped samples. Thus, they are expected to result in the decrease in capacity, depending on the dopant concentration. Although the 5% Ti + Mg sample shows better cyclability (94.3% up to 15 cycles) compared to 83.4, 87.5 and 93.4% for LiNiO2 , 1.25 and 2.5% Ti + Mg samples, respectively, its discharge capacity is lower. Compromising high specific capacity and good cyclability, the results suggest that the optimal dopant of Ti and Mg for LiNi0.85 Co0.15−2x (TiMg)x O2 is about x = 0.025. Fig. 6 shows the derivative dQ/dV (the first charge–discharge cycle) for the co-doped samples with different x. As shown, pure LiNiO2 characteristically exhibits many sharp peaks during the course of lithium intercalation-deintercalation, which have been thoroughly studied by in situ XRD and derivative studies on active electrochemical cells [19,20,22]. The sharp peaks are indicative of first-order phase transitions and two-phase co-existence and the broad ones are one-phase continuous phase transitions. During the charging process, the hexagonal phase changes from H1 to monoclinic (M) and then from M to H2 and finally H2 to H3 where H1, H2 and H3 represent phases with hexagonal symmetry but changed lattice parameters. During the discharging course, these phase transitions are reversible with a slight hysteresis. For Co-Ti-Mg co-doped samples, the shape of the derivative curves changes drastically, and the intensity of the peaks decreases with an increase of Ti and Mg doping content.
Fig. 6. The derivative dQ/dV of the first charge–discharge cycle for LiNi0.85 Co0.15−2x (TiMg)x O2 and LiNiO2 . H1, M1, H2 and H3 refer to different crystallographic phases.
This is suggestive that these transitions are suppressed. Thus, the change in derivative curves combined with XRD above, demonstrates that Co, Ti and Mg all are indeed incorporated into the structure, and this incorporation causes beneficial changes in the electrochemical cycling of lithium in the cathode material.
4. Conclusions Co-Ti-Mg co-doped samples of LiNi0.85 Co0.15−2x (TiMg)x O2 (x = 0.0125, 0.025 and 0.05) were synthesized by solid state reaction. Electrochemical studies showed that the co-doped samples have large beneficial effect on cycling reversibility compared with the pristine LiNiO2 . LiNi0.85 Co0.10 (TiMg)0.025 O2 give 182.7 mA h g−1 of the first discharge capacity, and the capacity can be largely remained after 15 cycles (retention ratio: 93.4%). Co, Ti and Mg (molar ratio Ti/Mg = 1.0) doped in LiNiO2 structure can suppress crystallographic phase transitions and decrease the irreversible capacity. Compromising high specific capacity and good cyclability, the LiNi0.85 Co0.10 (TiMg)0.025 O2 may be a good candidate cathode material for Li-ion batteries.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 29833090).
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