- Email: [email protected]
Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as anode material for lithium ion batteries: the influence of precursors
RARE METALS, Vol. 27, No. 2, Apr 2008, p. 170
Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as anode material for lithium ion batteries: the influence of precursors WEN Zhongshenga, b, TIAN Fenga, SUN Juncaia, JI Shijuna, and XIE Jingyingb a b
Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, China Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Received 20 January 2007; received in revised form 27 February 2007; accepted 15 March 2007
Abstract Ternary and quadruple lithium silicon nitride anode materials for lithium ion batteries with different precursors were prepared by the simple process of high-energy ball milling. High capacity and excellent cyclability were obtained. The influence of precursor introduction on the electrochemical performance of products was investigated. This research reveals that the electrochemical performance of lithium silicon nitride can be enhanced significantly by doping O. The cyclability of quadruple lithium silicon nitride can be optimized remarkably by controlling the introduction quantity of the precursors. It is possible for the composite to be used as a capacity compensator within a wide voltage cut-off window. Keywords: lithium ion batteries; high-capacity electrode material; high-energy ball milling; lithium silicon nitride; wide voltage cut-off window
1. Introduction To exploit new electrode materials with high capacity is indispensable to promote the performance of the current commercial lithium ion batteries with the rapid development of portable devices, electric vehicles (EV), and hybrid electric vehicles (HEV). Silicon, Sn and some alloy compounds that can alloy Li-ions with high ratios were exploited as promising high-capacity anode materials for lithium ion batteries in recent years [1-3], but seldom can get a long stable cyclability because of the structure-expanding and collapse caused by a considerable quantity of lithium-insertion and extraction occurring remarkably in the first cycle. The lithium transition metal nitride, as another important branch of high capacity anode materials distinguished from alloy compound for its special Li-loaded structure, shows different lithium-insertions and extraction behaviors during charge-discharge process. There is no serious capacity fading occurring for lithium transition metal nitride compounds during the first lithium insertion and extraction process although it also delivers a likewise high capacity [4-9]. Besides this, the most attractive characteristic of a lithium transition metal nitride is that it can be utilized as an additive to Corresponding author: WEN Zhongsheng
other lithium-free high capacity anode systems, such as silicon- or Sn-containing anode to compensate their irreversible capacity by providing extra lithium ion source [10-12]. The researches by Yang et al. demonstrated that the initial coulombic efficiency of SnO or SiO1.1 anode could approach to 100% by adding a small quantity of Li2.6Co0.4N to these electrode systems. It implies that the Li-loaded anode material might be a promising candidate to promote the electrochemical performance of high-capacity alloy or oxide compound anode materials by compensating their irreversibility [10-11]. However, the Li-loaded anode material is developed slowly to date and only lithium transition metal nitrides are widely researched and reported. With the development of high capacity anode and lithium-free cathode materials, to exploit a new suitable Li-loaded anode material is necessary. A tentative research extending Li-loaded anode materials from traditional lithium “transition metal” nitrides to lithium “non-transition metal” nitrides was carried out in the studies. New Li-loaded composite lithium silicon-containing nitrides, the capability of reversible Li insertion-extraction of which was found in the past studies [13-14], were synthesized with different precursors. In this article, the influence of precur-
E-mail: [email protected]
Wen Z.S. et al., Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as }
sors on the electrochemical performance was investigated in detail.
2. Experimental Planetary Mono Mill P-6 (Fritsch, Germany) was used for the synthesis of lithium silicon-containing nitride composites. The precursors were mixtures of Li3N powder (Aldrich) and silicon-containing powder. The product prepared from the mixture of Li3N and Si powder is lithium silicon nitride, and that from the mixture of Li3N and SiO2 powder is the O-doped one [13-14]. O-doped lithium silicon nitrides prepared from the precursors at different ratios of Li3N/SiO2 were examined in this study. The mole ratios of Li3N/SiO2 were 3/1, 4.2/1, 5.4/1, and the corresponding products are marked as L1, L2, and L3 in this article, respectively. The total milling time for the precursors was 10 h at a rotation speed of 500 r/min in N2 environment. Considering the deliquescence of the products, cells were assembled by the drying method as follows: 80 wt.% active material, 15 wt.% carbon black and 5 wt.% polytetrafluoroethylene (PTFE) binder were mixed homogeneously. The mixture was then pressed on a nickel foam disc (12.5 mm in diameter) with a pressure of about 0.3 Pa. CR2025 coin cells, containing 1 mol/L LiPF6/ethylence (EC)+dimethyl carbonate (DMC) (1:1 in volume) electrolyte and lithium foil as the opposite electrode, were assembled to evaluate the electrochemical properties of the active materials. Unless stated otherwise, the current density for charge and discharge was 0.3 mAcm2. The material is in lithium-rich state, so the cycling began with charging process, corresponding with lithium extraction.
171
traction of the both composite electrodes mainly occurs from open circuit voltage to 2.8 V, which is different from that of the followed cycles due to the structure transformation and collapse. After two cycles, the charge-discharge process becomes steady and assumes repeatable curves. The steady electrochemical process is attributed to the formation of the more steady structure after two cycles. The characteristic is thus similar to that of lithium transition metal nitrides [15-16]. However, compared with the latter, lithium silicon nitride exhibits a much higher voltage plateau and a wide voltage cut-off window during lithium inserting and extracting, because of different ingredient elements included in the structure. Also, the first extraction voltage plateau of O-doped lithium silicon nitride is a little higher than that of the before-doped ones, whereas in the subsequent cycles, these two composites show almost the same characteristics for electrochemical charging-discharging curves, due to their similar structure. The capability of charging-discharging with high capacity and high reversibility within a wide voltage cut-off windows makes them beneficial to compensating lithium ion sources of different electrode systems, either cathode or anode. So lithium silicon nitrides might be superior to any other Li-loaded material utilized as capacity compensators.
3. Results and discussion SEM photographs of the two composites synthesized from Li3N mixed with nano-Si and SiO2 powder have been presented in the earlier research study, as seen in Refs. [13-14]. The granularity of both is very fine with a wide span from submicrometer to micrometer within 3-5 Pm, which is beneficial to improving the electrochemical kinetics properties for the large specific surface and short ion-diffusion distance. The particle of the composite synthesized from Li3N mixed with nano-Si manifests a little more homogeneous but coarser than that with the precursor of SiO2 powder. The properties of the products are determined by the characteristics of different precursors. The past researches demonstrated that the composites synthesized from Li3N mixed with nano-Si and SiO2 powder were lithium silicon nitride and O-doped lithium silicon nitride (lithium silicon oxynitride), respectively [13-14]. Therefore, the electrochemical properties of these two composites are similar as shown in Fig. 1. The first lithium ex-
Fig. 1. Charge-discharge profiles of the composite electrode: (a) the 1st cycle; (b) the 2nd cycle.
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Fig. 2 shows the cycling performance of lithium silicon nitride and lithium silicon oxynitride electrodes at a current density of 0.3 mAcm2 within the wide cut-off windows shown in Fig. 1. Cycles under a wide voltage window can reflect the stability of the composite structure resisting a high-level lithium extraction. Both the composites exhibit an excellent cyclability. The initial discharge capacity of the former is 503.1 mAhg1 and the latter is 513 mAhg1. The capacity of lithium silicon nitride fades a little faster than that of oxynitride although with close initial discharge capacity. The specific capacity of the former remains above 400 mAhg1 after 50 cycles with 80% reversible capacity retention of the second cycle, whereas the latter is 488 mAhg1, 95% retention of the second cycle. Thus it is again demonstrated that the structure of lithium silicon nitride can be modified by doping O.
Fig. 2. Cycling performance of the composite electrode.
As the mentioned above, the composite of lithium silicon oxynitride can maintain a much better cyclability than lithium silicon nitride within a wide voltage cut-off window, so the influence of its precursors (SiO2 and Li3N powder) with different ratios on the electrochemical performance is investigated. The mole ratios of Li3N/SiO2 were 3/1, 4.2/1, 5.4/1, and the corresponding products are marked as L1, L2, and L3, respectively. The charge-discharge profiles of the products made from different mole ratios of precursors are presented in Fig. 1. The open circuit voltages of L1, L2, and L3 electrodes versus Li are 0.83 V, 1.25 V, and 1.12 V, respectively. The cell tests begin with charging process, which is corresponding with lithium extraction. There is a sloping voltage plateau on each first lithium extraction curve for the three products (Fig. 3(a)), which is corresponding to partial amorphous state of the products. L1 and L2 electrodes exhibit similar electrochemical characteristic for the first lithium extraction. Comparing with L1 and L2 electrodes, there is a voltage enhancement for L3 electrode toward positive direction for the first lithium extraction. It may be attributed
Fig. 3. Charge-discharge profiles of lithium silicon oxynitride composite electrodes between the cut-off voltage 0-3 V: (a) the 1st cycle; (b) the 2nd cycle; (c) the 5th cycle.
to the impurity introduced by excessive precursor. The voltage trend of the first lithium extraction is greatly different from that of the followed cycles. It is corresponding to the cyclic voltammograms of lithium silicon oxynitride. The sloping voltage plateau in the first charging process from open circuit voltage (OCV) to 3 V is substituted by a sloping curve from 0 to 3 V in the followed cycles (Figs. 3(b) and 3(c)). It presents a typical characteristic of amorphous structure. It can be deduced that the structure should be transformed to a more steady amorphous state immediately
Wen Z.S. et al., Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as }
after lithium extracting from the fresh electrode material and this state may be maintained in the followed cycles. The characteristic is similar to that of lithium transition metal nitrides. This is the main reason for the material possessing stable electrochemical performance after the first cycle. The discharge behavior of L3 is different from that of L1 and L2. A short voltage plateau between 0.8 V and 1.0 V appears on the discharge curves of L3 electrode after the first lithium extraction, as seen in Figs. 1(b) and 3(c). It may be attributed to different microstructures built from different ratios of precursors. Like any other lithium-loaded anode materials, the first extraction capacity of lithium silicic oxynitride electrode is much lower than its first insertions. One reason is related to the structure defects, such as lithium vacancy and dangling bonds, which can adopt additional lithium ions. Another reason is that the first lithium extracting begins with its open circuit voltage, so the extracting behavior under
173
OCV is not involved, and therefore the differences are created. The content of precursors plays an important role in the electrochemical cyclability of the final products. Table 1 represents the cycling performance of quadruple lithium silicon nitrides produced with different mole ratios of the precursors at the constant current density 0.3 mA/cm2 between the cut-off voltage 0-3 V. The first lithium extraction of L1 sample is about 683 mAhg1, which is the highest initial capability of the three samples, whereas it fades fast and presents a weak cyclability relatively. L2 and L3 exhibit a relatively excellent cyclability and there is almost no capability fading during 20-30 cycles. It is deduced that there exists some inherent relatives between the electrochemical performance of the final product with different precursors and the essential stoichiometric proportions in the structure.
Table 1. Cycling performance of quadruple lithium silicon nitrides at the constant current density of 0.3 mA/cm2 between the cut-off voltage 0-3 V Sample The mol ratio of the precursors (Li3N:SiO2) 1
L1
L2
L3
3.0:1
4.2:1
5.4:1
The first charging capacity / (mAhg )
471.3
429.5
329.1
The first discharging capacity / (mAhg1)
683.0
592.9
558.5
The first coloumbic efficiency (discharging capacity/charging capacity) / %
145
138
170
The charging capacity after 10 cycles / (mAhg1)
615.5
512.2
565.2
The charging capacity after 20 cycles / (mAhg1)
577.8
496.7
586.0
The charging capacity after 30 cycles / (mAhg1)
518.0
498.8
584.3
The cycling performance of L1, L2, and L3 samples is compared in Fig. 4. The given profiles are the discharge capacity of each sample, which demonstrate the effect of the mole ratio of the precursors on the electrochemical performance [17]. The result indicates that the mole ratio of the precursors plays an important role in the cycling performance of the product. As an important precursor for Li source of the final product, the introduction quantity of Li3N has a great effect on the capacity and the Li-ion diffusion rate. It seems more lithium vacancy can be produced inside in the structure of the product once less Li3N is added to the precursors. Therefore, the largest discharge capacity above 600 mAhg1 in the initial few cycles can be found for L1. However, its capacity fades rapidly for a considerable quantity of the irreversible lithium insertions. L3 presents a steady reversible capacity up to 590 mAhg1. A phenomenon of a gradual increase in the capacity with cycling continuing for L3 electrode was discovered. It is related to the incomplete reaction of the precursors. Although the initial specific capacity of L2 sample is only 592 mAhg1, which is lower than those of the other two samples, it exhibits the most ex-
cellent stability on cycling performance. Its specific capacity remains above 488 mAhg1 (95% retention of its second discharge capacity) after 50 cycles. The optimization of the electrochemical performance and deep understanding on this composite material is underway.
Fig. 4. Cycling performance of composite electrode between the cut-off voltage 0-3 V.
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4. Conclusions Novel lithium-rich anode materials for lithium ion batteries, lithium silicon nitride and oxynitride composites, were prepared by high-energy ball milling. The influence of precursors on the electrochemical performance was investigated in detail. The stability of the electrochemical performance of lithium silicon nitride can be enhanced significantly by doping O. Lithium silicon oxynitride produced at the precursors’ ratio of 4.2/1 (Li3N/SiO2) presents the best cyclability, and a capacity retention of 488 mAhg1 (95% of the second cycle) after 50 cycles can be attained. It is prior to many other high-capacity anode materials for lithium ion batteries. The capability of charging-discharging within a wide voltage cut-off windows makes them suitable to compensating lithium ion sources for different electrode systems, either cathode or anode.
Acknowledgements This study is financially supported by the National Natural Science Foundation of China (No. 50502009) and the Natural Science Foundation of Liaoning Province of China (No. 20072146).
References [1] Yang J., Wachtler M., Winter M., et al., Sub-microcrystalline Sn and Sn-SnSb powders as lithium storage materials for lithium-ion batteries, Electrochem. Solid-State Lett., 1999, 2: 161. [2] Beaulieu L.Y., Hewitt K.C., Turner R.L., et al., The Electrochemical reaction of Li with amorphous Si-Sn alloys, J. Electrochem. Soc., 2003, 150 (2): A149. [3] Limthongkul P., Jang Y., Dudney N.J., et al., Electrochemically-driven solid-state amorphization in lithium-silicon alloys and implications for lithium storage, Acta Mater., 2003, 51: 1103. [4] Nishijima M., Takeda Y., Imanishi N., et al., Li deintercalation and structural change in the lithium transition metal nitride Li3FeN2, J. Solid State Chem., 1994, 113: 205.
RARE METALS, Vol. 27, No. 2, Apr 2008 [5] Kim T.Y., Kim M.G., Lee J.M., et al., Local structural variations of Li2.6Co0.4N during the first charge and discharge, Electrochem. Solid State Lett., 2002, 5: A103. [6] Nishijima M., Kagohashi T., Imanishi M., et al., Synthesis and electrochemical studies of a new anode material, Li3˰ xCoxN, Solid State Ionics, 1996, 83: 107. [7] Liu Y., Horikawa K., Fujiyosi M., Imanishi N., et al., Layered lithium transition metal nitrides as novel anodes for lithium secondary batteries, Electrochim. Acta, 2004, 49: 3487. [8] Nishijima M., Takeda Y., Imanishi N., et al., Li deintercalation and structural change in the lithium transition metal nitride Li3FeN2, J. Solid State Chem., 1994, 113: 205. [9] Suzuki S. and Shodai T., Electronic structure and electrochemical properties of electrode material Li7xMnN4, Solid State Ionics, 1999, 116: 1. [10] Yang J., Takeda Y., Imanishi N., et al., Novel composite anodes based on nano-oxides and Li2.6Co0.4N for lithium ion batteries, Electrochim. Acta, 2001, 46: 2659. [11] Liu Y., Hanai K., Horikawa K., Imanishi N., et al., Electrochemical characterization of a novel Si-graphite-Li2.6Co0.4N composite as anode material for lithium secondary batteries, Mater. Chem. Phys., 2005, 89: 80. [12] Yang J., Takeda Y., Imanish N., et al., Tin-containing anode materials in combination with Li2.6Co0.4N for irreversibility compensation, J. Electrochem. Soc., 2000, 147: 1671. [13] Wen Z.S., Li S., Sun J.C., et al., Lithium silicic oxynitride composite as electrode material for lithium ion batteries, Electrochem. Solid State Lett., 2006, 9: A53. [14] Wen Z.S., Wang K., Chen L.B., et al., A new ternary composite lithium silicon nitride as anode materials for lithium ion batteries, Electrochem. Commun., 2006, 8: 1349. [15] Barker M.G. and Frankham S.A., The effects of carbon and nitrogen on the corrosion resistance of type 316 stainless steel to liquid lithium, J. Nucl. Mater., 1982, 107: 218. [16] Cabana J., Ling C.D., Oró-Solé J., et al., Anti-fluorite type lithium chromium oxynitrides: relationships among synthesis, structure, order and electrochemical properties, Inorg. Chem., 2004, 43: 7050. [17] Wen Z.S., Electrochemical studies of quadruple lithium silicic oxynitride as anode material for lithium ion batteries, Electrochem. Solid State Lett., 2007, 10: A21.
Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as anode material for lithium ion batteries: the influence of precursors WEN Zhongshenga, b, TIAN Fenga, SUN Juncaia, JI Shijuna, and XIE Jingyingb a b
Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, China Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
Received 20 January 2007; received in revised form 27 February 2007; accepted 15 March 2007
Abstract Ternary and quadruple lithium silicon nitride anode materials for lithium ion batteries with different precursors were prepared by the simple process of high-energy ball milling. High capacity and excellent cyclability were obtained. The influence of precursor introduction on the electrochemical performance of products was investigated. This research reveals that the electrochemical performance of lithium silicon nitride can be enhanced significantly by doping O. The cyclability of quadruple lithium silicon nitride can be optimized remarkably by controlling the introduction quantity of the precursors. It is possible for the composite to be used as a capacity compensator within a wide voltage cut-off window. Keywords: lithium ion batteries; high-capacity electrode material; high-energy ball milling; lithium silicon nitride; wide voltage cut-off window
1. Introduction To exploit new electrode materials with high capacity is indispensable to promote the performance of the current commercial lithium ion batteries with the rapid development of portable devices, electric vehicles (EV), and hybrid electric vehicles (HEV). Silicon, Sn and some alloy compounds that can alloy Li-ions with high ratios were exploited as promising high-capacity anode materials for lithium ion batteries in recent years [1-3], but seldom can get a long stable cyclability because of the structure-expanding and collapse caused by a considerable quantity of lithium-insertion and extraction occurring remarkably in the first cycle. The lithium transition metal nitride, as another important branch of high capacity anode materials distinguished from alloy compound for its special Li-loaded structure, shows different lithium-insertions and extraction behaviors during charge-discharge process. There is no serious capacity fading occurring for lithium transition metal nitride compounds during the first lithium insertion and extraction process although it also delivers a likewise high capacity [4-9]. Besides this, the most attractive characteristic of a lithium transition metal nitride is that it can be utilized as an additive to Corresponding author: WEN Zhongsheng
other lithium-free high capacity anode systems, such as silicon- or Sn-containing anode to compensate their irreversible capacity by providing extra lithium ion source [10-12]. The researches by Yang et al. demonstrated that the initial coulombic efficiency of SnO or SiO1.1 anode could approach to 100% by adding a small quantity of Li2.6Co0.4N to these electrode systems. It implies that the Li-loaded anode material might be a promising candidate to promote the electrochemical performance of high-capacity alloy or oxide compound anode materials by compensating their irreversibility [10-11]. However, the Li-loaded anode material is developed slowly to date and only lithium transition metal nitrides are widely researched and reported. With the development of high capacity anode and lithium-free cathode materials, to exploit a new suitable Li-loaded anode material is necessary. A tentative research extending Li-loaded anode materials from traditional lithium “transition metal” nitrides to lithium “non-transition metal” nitrides was carried out in the studies. New Li-loaded composite lithium silicon-containing nitrides, the capability of reversible Li insertion-extraction of which was found in the past studies [13-14], were synthesized with different precursors. In this article, the influence of precur-
E-mail: [email protected]
Wen Z.S. et al., Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as }
sors on the electrochemical performance was investigated in detail.
2. Experimental Planetary Mono Mill P-6 (Fritsch, Germany) was used for the synthesis of lithium silicon-containing nitride composites. The precursors were mixtures of Li3N powder (Aldrich) and silicon-containing powder. The product prepared from the mixture of Li3N and Si powder is lithium silicon nitride, and that from the mixture of Li3N and SiO2 powder is the O-doped one [13-14]. O-doped lithium silicon nitrides prepared from the precursors at different ratios of Li3N/SiO2 were examined in this study. The mole ratios of Li3N/SiO2 were 3/1, 4.2/1, 5.4/1, and the corresponding products are marked as L1, L2, and L3 in this article, respectively. The total milling time for the precursors was 10 h at a rotation speed of 500 r/min in N2 environment. Considering the deliquescence of the products, cells were assembled by the drying method as follows: 80 wt.% active material, 15 wt.% carbon black and 5 wt.% polytetrafluoroethylene (PTFE) binder were mixed homogeneously. The mixture was then pressed on a nickel foam disc (12.5 mm in diameter) with a pressure of about 0.3 Pa. CR2025 coin cells, containing 1 mol/L LiPF6/ethylence (EC)+dimethyl carbonate (DMC) (1:1 in volume) electrolyte and lithium foil as the opposite electrode, were assembled to evaluate the electrochemical properties of the active materials. Unless stated otherwise, the current density for charge and discharge was 0.3 mAcm2. The material is in lithium-rich state, so the cycling began with charging process, corresponding with lithium extraction.
171
traction of the both composite electrodes mainly occurs from open circuit voltage to 2.8 V, which is different from that of the followed cycles due to the structure transformation and collapse. After two cycles, the charge-discharge process becomes steady and assumes repeatable curves. The steady electrochemical process is attributed to the formation of the more steady structure after two cycles. The characteristic is thus similar to that of lithium transition metal nitrides [15-16]. However, compared with the latter, lithium silicon nitride exhibits a much higher voltage plateau and a wide voltage cut-off window during lithium inserting and extracting, because of different ingredient elements included in the structure. Also, the first extraction voltage plateau of O-doped lithium silicon nitride is a little higher than that of the before-doped ones, whereas in the subsequent cycles, these two composites show almost the same characteristics for electrochemical charging-discharging curves, due to their similar structure. The capability of charging-discharging with high capacity and high reversibility within a wide voltage cut-off windows makes them beneficial to compensating lithium ion sources of different electrode systems, either cathode or anode. So lithium silicon nitrides might be superior to any other Li-loaded material utilized as capacity compensators.
3. Results and discussion SEM photographs of the two composites synthesized from Li3N mixed with nano-Si and SiO2 powder have been presented in the earlier research study, as seen in Refs. [13-14]. The granularity of both is very fine with a wide span from submicrometer to micrometer within 3-5 Pm, which is beneficial to improving the electrochemical kinetics properties for the large specific surface and short ion-diffusion distance. The particle of the composite synthesized from Li3N mixed with nano-Si manifests a little more homogeneous but coarser than that with the precursor of SiO2 powder. The properties of the products are determined by the characteristics of different precursors. The past researches demonstrated that the composites synthesized from Li3N mixed with nano-Si and SiO2 powder were lithium silicon nitride and O-doped lithium silicon nitride (lithium silicon oxynitride), respectively [13-14]. Therefore, the electrochemical properties of these two composites are similar as shown in Fig. 1. The first lithium ex-
Fig. 1. Charge-discharge profiles of the composite electrode: (a) the 1st cycle; (b) the 2nd cycle.
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Fig. 2 shows the cycling performance of lithium silicon nitride and lithium silicon oxynitride electrodes at a current density of 0.3 mAcm2 within the wide cut-off windows shown in Fig. 1. Cycles under a wide voltage window can reflect the stability of the composite structure resisting a high-level lithium extraction. Both the composites exhibit an excellent cyclability. The initial discharge capacity of the former is 503.1 mAhg1 and the latter is 513 mAhg1. The capacity of lithium silicon nitride fades a little faster than that of oxynitride although with close initial discharge capacity. The specific capacity of the former remains above 400 mAhg1 after 50 cycles with 80% reversible capacity retention of the second cycle, whereas the latter is 488 mAhg1, 95% retention of the second cycle. Thus it is again demonstrated that the structure of lithium silicon nitride can be modified by doping O.
Fig. 2. Cycling performance of the composite electrode.
As the mentioned above, the composite of lithium silicon oxynitride can maintain a much better cyclability than lithium silicon nitride within a wide voltage cut-off window, so the influence of its precursors (SiO2 and Li3N powder) with different ratios on the electrochemical performance is investigated. The mole ratios of Li3N/SiO2 were 3/1, 4.2/1, 5.4/1, and the corresponding products are marked as L1, L2, and L3, respectively. The charge-discharge profiles of the products made from different mole ratios of precursors are presented in Fig. 1. The open circuit voltages of L1, L2, and L3 electrodes versus Li are 0.83 V, 1.25 V, and 1.12 V, respectively. The cell tests begin with charging process, which is corresponding with lithium extraction. There is a sloping voltage plateau on each first lithium extraction curve for the three products (Fig. 3(a)), which is corresponding to partial amorphous state of the products. L1 and L2 electrodes exhibit similar electrochemical characteristic for the first lithium extraction. Comparing with L1 and L2 electrodes, there is a voltage enhancement for L3 electrode toward positive direction for the first lithium extraction. It may be attributed
Fig. 3. Charge-discharge profiles of lithium silicon oxynitride composite electrodes between the cut-off voltage 0-3 V: (a) the 1st cycle; (b) the 2nd cycle; (c) the 5th cycle.
to the impurity introduced by excessive precursor. The voltage trend of the first lithium extraction is greatly different from that of the followed cycles. It is corresponding to the cyclic voltammograms of lithium silicon oxynitride. The sloping voltage plateau in the first charging process from open circuit voltage (OCV) to 3 V is substituted by a sloping curve from 0 to 3 V in the followed cycles (Figs. 3(b) and 3(c)). It presents a typical characteristic of amorphous structure. It can be deduced that the structure should be transformed to a more steady amorphous state immediately
Wen Z.S. et al., Electrochemical characteristics of ternary and quadruple lithium silicon nitrides as }
after lithium extracting from the fresh electrode material and this state may be maintained in the followed cycles. The characteristic is similar to that of lithium transition metal nitrides. This is the main reason for the material possessing stable electrochemical performance after the first cycle. The discharge behavior of L3 is different from that of L1 and L2. A short voltage plateau between 0.8 V and 1.0 V appears on the discharge curves of L3 electrode after the first lithium extraction, as seen in Figs. 1(b) and 3(c). It may be attributed to different microstructures built from different ratios of precursors. Like any other lithium-loaded anode materials, the first extraction capacity of lithium silicic oxynitride electrode is much lower than its first insertions. One reason is related to the structure defects, such as lithium vacancy and dangling bonds, which can adopt additional lithium ions. Another reason is that the first lithium extracting begins with its open circuit voltage, so the extracting behavior under
173
OCV is not involved, and therefore the differences are created. The content of precursors plays an important role in the electrochemical cyclability of the final products. Table 1 represents the cycling performance of quadruple lithium silicon nitrides produced with different mole ratios of the precursors at the constant current density 0.3 mA/cm2 between the cut-off voltage 0-3 V. The first lithium extraction of L1 sample is about 683 mAhg1, which is the highest initial capability of the three samples, whereas it fades fast and presents a weak cyclability relatively. L2 and L3 exhibit a relatively excellent cyclability and there is almost no capability fading during 20-30 cycles. It is deduced that there exists some inherent relatives between the electrochemical performance of the final product with different precursors and the essential stoichiometric proportions in the structure.
Table 1. Cycling performance of quadruple lithium silicon nitrides at the constant current density of 0.3 mA/cm2 between the cut-off voltage 0-3 V Sample The mol ratio of the precursors (Li3N:SiO2) 1
L1
L2
L3
3.0:1
4.2:1
5.4:1
The first charging capacity / (mAhg )
471.3
429.5
329.1
The first discharging capacity / (mAhg1)
683.0
592.9
558.5
The first coloumbic efficiency (discharging capacity/charging capacity) / %
145
138
170
The charging capacity after 10 cycles / (mAhg1)
615.5
512.2
565.2
The charging capacity after 20 cycles / (mAhg1)
577.8
496.7
586.0
The charging capacity after 30 cycles / (mAhg1)
518.0
498.8
584.3
The cycling performance of L1, L2, and L3 samples is compared in Fig. 4. The given profiles are the discharge capacity of each sample, which demonstrate the effect of the mole ratio of the precursors on the electrochemical performance [17]. The result indicates that the mole ratio of the precursors plays an important role in the cycling performance of the product. As an important precursor for Li source of the final product, the introduction quantity of Li3N has a great effect on the capacity and the Li-ion diffusion rate. It seems more lithium vacancy can be produced inside in the structure of the product once less Li3N is added to the precursors. Therefore, the largest discharge capacity above 600 mAhg1 in the initial few cycles can be found for L1. However, its capacity fades rapidly for a considerable quantity of the irreversible lithium insertions. L3 presents a steady reversible capacity up to 590 mAhg1. A phenomenon of a gradual increase in the capacity with cycling continuing for L3 electrode was discovered. It is related to the incomplete reaction of the precursors. Although the initial specific capacity of L2 sample is only 592 mAhg1, which is lower than those of the other two samples, it exhibits the most ex-
cellent stability on cycling performance. Its specific capacity remains above 488 mAhg1 (95% retention of its second discharge capacity) after 50 cycles. The optimization of the electrochemical performance and deep understanding on this composite material is underway.
Fig. 4. Cycling performance of composite electrode between the cut-off voltage 0-3 V.
174
4. Conclusions Novel lithium-rich anode materials for lithium ion batteries, lithium silicon nitride and oxynitride composites, were prepared by high-energy ball milling. The influence of precursors on the electrochemical performance was investigated in detail. The stability of the electrochemical performance of lithium silicon nitride can be enhanced significantly by doping O. Lithium silicon oxynitride produced at the precursors’ ratio of 4.2/1 (Li3N/SiO2) presents the best cyclability, and a capacity retention of 488 mAhg1 (95% of the second cycle) after 50 cycles can be attained. It is prior to many other high-capacity anode materials for lithium ion batteries. The capability of charging-discharging within a wide voltage cut-off windows makes them suitable to compensating lithium ion sources for different electrode systems, either cathode or anode.
Acknowledgements This study is financially supported by the National Natural Science Foundation of China (No. 50502009) and the Natural Science Foundation of Liaoning Province of China (No. 20072146).
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