Elsevier

Energy Storage Materials

Volume 19, May 2019, Pages 352-359
Energy Storage Materials

High temperature and high rate lithium-ion batteries with boron nitride nanotubes coated polypropylene separators

https://doi.org/10.1016/j.ensm.2019.03.027Get rights and content

Abstract

Safety of lithium-ion batteries (LIBs) is a current serious and challenging issue threating large-scale energy storage application as well as every day usage of mobile devices. Initial overheating of the cell is one of the factors responsible for the failure of LIB safety, which is caused by short circuit under high temperature and high current environment. As separator is the main component to prevent short circuit, thermal stability of the separator is crucial in this regard. In this study, boron nitride nanotubes (BNNTs) are synthesized and used for the first time as a new type of high performance inorganic nanomaterials to prevent short-circuit. We provide a new configuration strategy for the modification of conventional polyolefin separator by simply incorporation of appropriately engineered long and fine BNNTs without blocking the porous channels of the conventional separator for Li+ ion diffusion. This new BNNT separator exhibits improved thermal stability up to 150 °C, ensuring the safe operation of LIB cells at elevated temperatures. The high rate capability of the cell with BNNT separator is also improved dramatically due to absorb extra heat and spread it through BNNTs during the cycling process. The BNNTs demonstrate an exciting new nanomaterials in improving thermal stability of the polyolefin separator by protecting thermal shrinkage at high temperature and high current operation, which eventually prevent battery short-circuit.

Introduction

Improving battery technology is a high priority as the world seeks to make more efficient use of energy resources. Lithium-ion battery (LIB) technology is playing a major role in powering high energy and power density devices on a larger scale for more demanding applications, such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and electric power grids [[1], [2], [3]]. However, a serious and challenging issue is battery safety, particularly the risk of batteries catching fire and explosion. In this context, few examples such as battery fire and explosion of the Samsung Note 7, battery fire of the Tesla electric car, and battery problem of the Boeing 787 Dreamliner, etc. have raised existing safety concerns of LIBs at an extreme level [4]. As a result, these serious safety issues with LIBs need to be resolved before their high power applications in the future. To solve the safety issue with LIBs, need to understand safety methods, which include external and internal protection mechanisms. Internal protection mechanism is critical as the failure of this mechanism is the main reason for the safety concerns, which depend on materials for battery component.

A LIB consists of three functional components: i) electrode (anode and cathode), ii) separator, and iii) electrolyte. Among them, the separators, which are placed between the two electrodes to prevent their electric contact while allowing ionic transport, could be a weak point because of poor thermal stability [5,6]. The failures of safety of LIBs are mainly due to thermal runaway because of overheating [[7], [8], [9], [10]]. A number of factors cause the initial overheating where both high external temperature as well as excessive currents at high rate charge-discharge could be considered as critical factors. Under such extreme environment, commercial polyolefin separators start to shrink which eventually initiates short circuit of the cell. Initial short circuit accelerates uncontrollable exothermic reaction, which decomposes solid electrolyte interface layers, electrolyte and electrodes and release a lot of heat, even higher than 200 °C. As a result, serious thermal runaway causes fire and explosion [7,10]. The underlying problem with the commercial polyolefin separators (including polyethylene, PE and polypropylene, PP) is their low melting point (melting points of ∼135 °C for PE and ∼165 °C for PP) [7,10,11]. A significant dimensional instability has been observed with polyolefin separators at elevated operating temperature, in fact, it is happened at a low temperature of ∼65 °C when cell is operated at a high current rate, which potentially initiates an internal short circuit [[12], [13], [14]]. As separator is the main component to prevent short circuit, therefore, thermal stability of the separator is crucial.

To enhance thermal stability of the LIB separators, many safe materials have been discovered and used for the modification of commercial polyolefin separators. Modification of polyolefin separators through coating with various safe inorganic nanoparticles (such as SiO2, Al2O3, Al (OH)3, Mg (OH)2, Zeolite, ZrO2, and TiO2) is considered one of the most effective and economic approaches to enhance thermal stability of the conventional polyolefin separators [5,12,[15], [16], [17]]. However, other important parameters such as mechanical stability and cycling performance at high current rates and high temperatures are not well balanced with these separators. The modified separators with inorganic nanoparticles suffer from severe low flexibility (mechanically unstable) as they are brittle, causes cracks during cell assembling, facilitates immediate short circuits during cell operation. On the other hand, exploration of alternatives to polyolefin separators, specifically non-woven separators have been developed. Even though non-woven separators exhibit superior thermal stability, the serious problems with these separators are their large pores, which induce internal short circuits, self-discharge, and significant electrolyte leakage [18,19]. Hence, separators with an excellent balance of security-reliability-performance are in urgent demand for the advancement of safe LIBs, which is a current challenge and difficult to achieve. In this regard, discovery of safe nanomaterials for separator with desire characteristics is crucial to achieve an excellent balance of security-reliability-performance with LIBs.

In this study, we discover that boron nitride nanotubes (BNNTs) can potentially be used as a high performance safe nanomaterial for the improvement of LIB safety under extreme environment. Although previously boron nitride nanosheets has been incorporated into a polyolefin separator to inhibit dendritic lithium development during cycling, however, high temperature-high current operation have not been investigated [20]. We provide a new configuration strategy for the modification of polyolefin separator by simply incorporation of appropriately engineered BNNTs (rather than nanosheets), as illustrated in Fig. 1. Appropriately engineered BNNTs can be viewed as hexagonal boron nitride (h-BN) rolled up into seamless tubes with nanosized diameter along with high thermal conductivity, high temperature and mechanical stability [19,[21], [22], [23], [24]]. Most importantly, long and fine BNNTs cannot block the porous channels of the separators for Li+ ion diffusion. Due to these unique properties, BNNTs might be better safe nanomaterials than any other inorganic nanomaterials for improving rate capability and thermal stability of the separators in LIBs for high temperatures and high current operation. The obtained new BNNT separators are capable to: i) enhance thermal stability of the separators up to 150 °C; ii) improve electrolyte wettability, which is favourable for high rate capability; and iii) improve electrochemical performance, particularly at high temperature and high charge/discharge current rates. This work highlights the importance of battery safety and provides a potential solution to creditably decrease the threat of thermal runaway of LIBs by preventing internal short circuits at elevated temperatures by allowing an innovative application of BNNTs in battery technology.

Section snippets

Synthesis of BNNTs

BNNTs were synthesized according to the procedure described in our previous work [21]. Fine boron (B) particles were first ball milled in anhydrate ammonia (NH3) gas to produce nanosized B particles, the milled B powders were subsequently heated in a horizontal tube furnace at 1100 °C for several hours in nitrogen gas to produce BNNTs.

Fabrication of BNNT separator (one side coated)

Suitable amount of binder of PVA (poly (vinyl alcohol)) was mixed in 50 ml deionized (DI) water and heated at 90 °C under continuous stirring for 1 h. 0.5 g of

Results and discussion

Thermal stability of the BNNT separators was tested and compared with the commercial Celgard (PP) separator. According to the results of differential scanning calorimetry (DSC) as shown in Fig. 2a, the pronounced exothermic peak of polypropylene (PP) related to the melting temperature of the material indicates that the PP separator experienced total melt down at 165 °C, whereas the BNNT separator (double side coated) possessed much improved thermal stability as a much smaller exothermic peak at

Conclusions

In summary, fine and ultra long boron nitride nanotubes (BNNTs) with several hundred micrometres in length and ∼50–55 nm in diameter are synthesized and used for the first time as a safe nanomaterial to prevent internal short-circuit of the LIB cells at high temperature and high current operation. A new BNNT separator is successfully fabricated through incorporation of thermally and chemically stable BNNTs onto polyolefin (PP) separators via a dip coating process. The new BNNT separator

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work is supported by Australian Research Council LP170100784 grant. Deakin University's Advanced Characterization Facilities are also acknowledged.

References (40)

  • Y. Chen et al.

    Influence of milling temperature and atmosphere on the synthesis of iron nitrides by ball milling

    Mater. Sci. Eng.

    (1996)
  • M.D. Levi et al.

    The mechanism of lithium intercalation in graphite film electrodes in aprotic media. part 1. High resolution slow scan rate cyclic voltammetric studies and modeling

    J. Electroanal. Chem.

    (1997)
  • G. Leftheriotis et al.

    Dependence of the estimated diffusion coefficient of LixWO3 films on the scan rate of cyclic voltammetry experiments

    Solid State Ionics

    (2007)
  • M. Armand et al.

    Building better batteries

    Nature

    (2008)
  • V. Etacheri et al.

    Challenges in the development of advanced Li-ion batteries: a review

    Energy Environ. Sci.

    (2011)
  • N. Kamaya et al.

    A lithium superionic conductor

    Nat. Mater.

    (2011)
  • K. Liu et al.

    Materials for lithium-ion battery safety

    Sci. Adv.

    (2018)
  • P. Arora et al.

    Battery separators

    Chem. Rev.

    (2004)
  • J.C. Orendorff et al.

    Polyester separators for lithium-ion cells: improving thermal stability and abuse tolerance

    Adv. Energy Mater.

    (2013)
  • L. Yu et al.

    A comparative study on polypropylene separators coated with different inorganic materials for lithium-ion batteries

    Front. Chem. Sci. Eng.

    (2017)
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