Fabrication of 5 v lithium rechargeable micro-battery

Journal of Power Sources 132 (2004) 240–243 Fabrication of 5 V lithium rechargeable micro-battery Electrochemical Research Center, P.O. Box 19395-5139, Tehran 15875-4416, Iran Received 11 October 2003; received in revised form 11 December 2003; accepted 2 January 2004 Abstract
A 5 V lithium secondary cell was fabricated using LiFe0.5Mn1.5O4 cathode material with all-solid-state design. As an additional approach to improve the battery performance of the all-solid-state secondary cell, a metal oxide-retaining layer was placed between the cathodematerial and solid electrolyte. The cathode material has an excellent cyclability in this cell design, since it just loses 5 and 18% of its initialcapacity after 100 and 500 charge–discharge cycles, respectively.
2004 Elsevier B.V. All rights reserved.
Keywords: 5 V battery; Lithium secondary cell; Lipon; All-solid-state cell; 5 V cathode; LiFe0.5Mn1.5O4 1. Introduction
There have been numerous efforts to improve battery per-formance of LiMn2O4 as a promising alternative to current Recent developments of cathodes of lithium secondary LiCoO2 cathodes. This is due to noticeable advantages cells have introduced 5 V cathode materials. Up to now, they of LiMn2O4 in comparison with other cathode materials, have not been used for the fabrication of lithium secondary such as abundance of Mn sources, non-toxicity, low-cost, cells, and indeed technology of lithium secondary cells is etc. Thus, LiM0.5Mn1.5O4 spinels as 5 V cathode materials still restricted to 4 V rechargeable batteries. This is due to are of double interest. In this class of 5 V cathode mate- the lack of sufficient improvement of these cathode materi- rials, LiFe0.5Mn1.5O4 has some advantages such as lower als to gain acceptable battery performance for the practical cost of Fe in comparison with other substituted metals.
applications. The preliminary investigations of 5 V cathode In the previous paper we have extensively studied materials ve shown that the main obstacle in use of the diffusion process of Li+ intercalation–deintercalation these cathode materials is their high-voltage performance, into/from LiFe0.5Mn1.5O4 spinel in different media. It has which is accompanied by severe problems. Our recent at- been demonstrated that the problems of high-voltage perfor- tempts this context have significantly overcome mance is mainly related to the direct reaction of electrolyte this problem and have provided a new opportunity for the salt with the high-valent metals of the cathode materials fabrication of 5 V lithium secondary cells. In the present at high-voltage performance. Thus, capacity fading of the communication, possibility for the fabrication of 5 V lithium cathode material can be significantly reduced by choosing secondary cells is reported. To this aim, an all-solid-state cell appropriate electrolyte medium It is worth noting design was used, which reduces problems of high-voltage that using different substituting metals leads to different performance and on the other hand provides an oppor- advantages of the LiFe0.5Mn1.5O4 spinel. For instance, tunity for the fabrication of high-voltage rechargeable LiNi0.5Mn1.5O4 provides a single plateau during the elec- trochemical performance due to its single redox, with rela- The most important class of 5 V cathode materials of tively high-specific capacity. LiCu0.5Mn1.5O4 is also known lithium secondary cells is LiM0.5Mn1.5O4 (M: Ni, Co, Cr, as a metal-substituted LiMn2O4 with a negligible capac- Cu, Fe, etc). In addition to 5 V performance of this class of ity fading due to its crystallographic stability. However, cathode materials, they have essential advantages as well each of them suffers from some important disadvantages as LiMn2O4 cathodes of 4 V lithium secondary batteries.
such as toxicity, expensivity, etc. Overall, advantages ofLiFe0.5Mn1.5O4 guarantee the practical performance.
To fabricate a promising secondary cell, a more appro- ∗ Tel.: +98-21-204-2549; fax: +98-21-205-7621.
priate electrolyte medium viz. Lipon, which is a solid elec- E-mail address: eftekhari@elchem.org (A. Eftekhari).
trolyte of lithium secondary cells was used in the present 0378-7753/$ – see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2004.01.001 A. Eftekhari / Journal of Power Sources 132 (2004) 240–243 research. Lithium phosphorous oxynitride (Lipon) is anexcellent electrolyte for solid-state lithium thin-film batter-ies due to their significant electrochemical stability(5.5 V stability window versus Li/Li+) and high ionic con-ductivity. Interestingly, this design provides an opportunityfor the fabrication of thin-film battery (TFB), which isa promising alternative micropower source satisfying theneed to derive extremely small size electronic devices ofthe current era (and indeed are quickly getting smaller).
2. Experimental
The synthesis procedure for the preparation of LiFe0.5 Mn1.5O4 spinel has been described in the literature t(150 nm)/Ti (20 nm) current collector layers were deposited Fig. 1. Typical charge–discharge characteristic of the 5 V lithium sec- on Si(1 0 0) wafer by a sputtering method. The titanium layer ondary cell with all-solid-state design. Rate: C/10.
was used for good adhesion between the Pt current collectorand silicon substrate. The cathode material LiFe0.5Mn1.5O4 that the thin-film cathode in solid-state cell has its original was deposited onto the current collector by radio frequency electrochemical behavior. However, the main problem in (rf) magnetron sputtering. A rf power of 100 W under an the preparation of 5 V lithium secondary batteries in the Ar atmosphere with 1.4 Pa pressure using a conventional appearance of significant capacity fades in the course of rf magnetron sputtering system was employed for this pur- cycling. Thus, to fabricate a 5 V lithium secondary battery, pose. The cathode film thickness was about 500 nm, and it is needed to obtain good cyclability.
the amount of electroactive material attached to the sub- The results obtained from repetitive charging–discharging strate surface was about 80 ␮g. The solid electrolyte, Lipon, of the 5 V lithium secondary cell are presented in It was deposited by rf magnetron sputtering of a Li3PO4 tar- is obvious that the cyclability of the all-solid-state lithium get in a nitrogen atmosphere The Lipon electrolyte secondary cell using Lipon electrolyte is significantly higher thickness was about 1 ␮m, with ionic conductivity of about than that of previously reported using conventional LiPF6 2 × 10−6 S/cm. Finally, the lithium metal film was deposited electrolyte salt and even stable electrolyte salt of LiBF4 over the Lipon electrolyte by conventional thermal evapo- The 5 V lithium secondary cell fabricated exhibits ration using a small evaporator set up in an Ar-filled glove only about 10% capacity fade after 100 charge–discharge cycles. The results obtained from cyclability investigation Two different all-solid-state secondary cells were pre- of the all-solid-state cell with Al2O3-retaining layer are pared and compared. A so-called conventional secondary also reported in comparison. The latter secondary cell was fabricated using simple LiFe0.5MnO1.5O4 cathode,and the other using a metal oxide-coated LiFe0.5MnO4 cath-ode. Aluminum oxide was used as a typical metal oxide as aretaining layer which avoids direct contact of the electroac-tive film and electrolyte solution. A thin layer of Al2O3 withthickness of ca. 10 nm was deposited onto the electroactivefilm by sputtering method. Charge–discharge tests were car-ried out using a battery cycler with C/10 rate.
3. Results and discussion
In the previous paper battery performance of LiFe0.5Mn1.5O4 cathode in liquid electrolytes has been ex-tensively studied. In this direction, it is aimed to investigatethe battery performance of this 5 V cathode in all-solid-statedesign. ws a typical charge–discharge character-istic of the 5 V lithium secondary cell with an all-solid-state Fig. 2. Cyclability data for: the conventional 5 V lithium secondary cellusing a conventional LiFe0.5Mn1.5O4 cathode ((ᮀ) charge and (᭿) dis- design described in The curve is similar to that charge); and using a Al2O3-coated LiFe0.5Mn1.5O4 cathode ((᭺) charge reported for the battery performance of LiFe0.5Mn1.5O4 and (᭹) discharge). The charge–discharge profiles were recorded as in cathode in liquid electrolyte medium This suggests A. Eftekhari / Journal of Power Sources 132 (2004) 240–243 cell exhibits an excellent battery performance, since it losesonly 5% of its initial capacity after 100 cycles during 5 Vperformance.
The values corresponding to the charge capacity are also reported in Since the capacity fading is stronger inthe course of discharge in comparison with charging, the gapbetween the charge and discharge capacity increases uponcycling. In fact, charge capacity decreases due to weakerLi diffusion resulting in more incomplete insertion. Since alesser amount of Li is inserted, a lesser capacity is expectedduring discharging. Thus, the discharge capacity decreasesas well as the charge capacity, plus an additional capacityfading (in addition to the mentioned capacity fading due tolesser amount of Li in LixMn2O4) due to other side reactionsmainly electrolyte decomposition. This failure is only appro-priate for the discharging process, since electrolyte decom- Fig. 3. Cyclability data for 4 V performance of: (᭹) the conventional position occurs at high-voltage performance of discharging, all-solid-state secondary cell and (᭺) the all-solid-state secondary cell which is very strong for 5 V cathodes. The main advantage using Al2O3-retaining layer. The charge–discharge cycles were recordedin potential window of 3.0–4.4 with C/10 rate.
of the all-solid-state cell is to reduce electrolyte decompo-sition due to the absence of liquid electrolyte. Therefore, itis expected to achieve a lesser gap between the charge and at elevated temperature, which is due to Mn dissolution.
discharge capacity. This behavior can be observed by com- Interestingly, it is observable that surface coating of the parison of the results reported in and corresponding LiFe0.5Mn1.5O4 also improves its 4 V performance, because gaps appeared in two different cells.
avoiding the direct contact of the cathode with electrolyte For the conventional all-solid-state cell, this gap de- has an effect to reduce possible Mn dissolution (though it creases during the initial cycles up to 10th to reach a min- is significantly less in metal-substituted LiMn2O4). This is imum, and then increases. The gradual increase of the gap due to the metal oxide-retaining layer which avoids surface is expected since the discharge capacity fading is stronger structural changes of the cathode material.
at high cycle numbers, but the initial decrease of the gap is Although, this is an acceptable cyclability for this new extraordinary. The latter behavior can be attributed to the class of lithium secondary cells (i.e. 5 V rechargeable batter- modification of surface structure appearing during the initial ies), further improvements in cyclability of such 5 V lithium cycles. Note that all-solid-state design does not guarantee secondary cell is required for practical applications and com- a perfect contact of the rigid electroactive material with the mercialization, since higher numbers of charge–discharge solid electrolyte. However, this behavior is not observed for cycles is desired for applied purposes. As stated above, the the battery performance of the LiFe0.5Mn1.5O4 cathode in main reason for capacity fading of 5 V cathode materials due liquid electrolyte, since the electrolyte fills empty pores of to high-voltage performance is related to the direct reaction the cathode material and makes a perfect contact at the elec- of electrolyte and cathode material at the cathode–electrolyte troactive film–electrolyte solution interface. Interestingly, it interface. This has a great importance for LiMn2O4 and sim- is observable in this failure also disappears for ilar compounds, e.g. LiM0.5Mn1.5O4 as they have very com- the all-solid-state cell with Al2O3-retaining layer, since the plex surfaces which are subject of significant surface metal oxide is less rigid in comparison with the electroac- changes upon cycling it is possible to reduce such tive material and also avoids subsequent changes in the capacity fades by avoiding such direct contact or even incor- surface structure upon cycling. This is another advantage poration of metal oxide into the electroactive film (to cover of this metal oxide-retaining layer.
the individual particle surfaces) It has been reported To investigate that the capacity fading appeared for the as well as LiMn2O4 cathodes, surface coating of LiFe0.5Mn1.5O4 is mainly due to the high-voltage perfor- 5 V cathode materials is an efficient approach to reduce mance of the cathode, it is needed to inspect the capac- capacity fading. Thus, surface coating the electroactive film ity fading in the course of conventional 4 V performance with a retaining layer is useful to gain battery performance (It is obvious that the capacity fading appearing in in the course of more cycling. Further investigations in this the course of 4 V performance is negligible. This suggests context showed that this cell can be successfully used for that the common Mn dissolution of LiMn2O4 is not re- 500 charge–discharge cycles with only 18% capacity fades sponsible for strong capacity fading of LiFe0.5Mn1.5O4 5 V (not shown). This is an acceptable cyclability for this new cathode, since it is known that substituting transition metal class of lithium secondary batteries, though, it is just prelim- into LiMn2O4 spinel can reduces Mn dissolution. Indeed, it inary investigations in this direction and future researches was the main reason for investigation of metal-substituted will surely provide significantly better cyclability and LiMn2O4: to reduce capacity fading of LiMn2O4 cathodes A. Eftekhari / Journal of Power Sources 132 (2004) 240–243 4. Conclusion
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