Nickel–metal hydride battery

: Work on NiMH batteries began at the Battelle-Geneva Research Center following the technology's invention in 1967. It was based on sintered Ti2Ni+TiNi+x alloys and NiOOH electrodes. Development was sponsored over nearly two decades by Daimler-Benz and by Volkswagen AG within Deutsche Automobilgesellschaft, now a subsidiary of Daimler AG. The batteries' specific energy reached 50 W·h/kg (180 kJ/kg), specific power up to 1000 W/kg and a life of 500 charge cycles (at 100% depth of discharge). Patent applications were filed in European countries (priority: Switzerland), the United States, and Japan. The patents transferred to Daimler-Benz. Interest grew in the 1970s with the commercialisation of the nickel–hydrogen battery for satellite applications. Hydride technology promised an alternative, less bulky way to store the hydrogen. Research carried out by Philips Laboratories and France's CNRS developed new high-energy hybrid alloys incorporating rare-earth metals for the negative electrode. However, these suffered from alloy instability in alkaline electrolyte and consequently insufficient cycle life. In 1987, Willems and Buschow demonstrated a successful battery based on this approach (using a mixture of La0.8Nd0.2Ni2.5Co2.4Si0.1), which kept 84% of its charge capacity after 4000 charge-discharge cycles. More economically viable alloys using mischmetal instead of lanthanum were soon developed. Modern NiMH cells were based on this design. The first consumer-grade NiMH cells became commercially available in 1989. In 1998, Stanford Ovshinsky at Ovonic Battery Co., which had been working on MH-NiOOH batteries since mid-1980, improved the Ti–Ni alloy structure and composition and patented its innovations. In 2008, more than two million hybrid cars worldwide were manufactured with NiMH batteries. In the European Union due to its Battery Directive, nickel–metal hydride batteries replaced Ni–Cd batteries for portable consumer use. In Switzerland in 2009, approximately 60% of portable rechargeable batteries were NiMH. In 2000, almost half of all portable rechargeable batteries sold in Japan were NiMH, This percentage has fallen over time due to the increase in manufacture of lithium-ion batteries. In 2015 BASF produced a modified microstructure that helped make NiMH batteries more durable, in turn allowing changes to the cell design that saved considerable weight, allowing the specific energy to reach 140 watt-hours per kilogram. == Electrochemistry ==

The negative electrode reaction occurring in a NiMH cell is : H2O + M + e− OH− + MH On the positive electrode, nickel oxyhydroxide, NiO(OH), is formed: : Ni(OH)2 + OH− NiO(OH) + H2O + e− The reactions proceed left to right during charge and the opposite during discharge. The metal M in the negative electrode of a NiMH cell is an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a rare-earth mixture of lanthanum, cerium, neodymium, praseodymium, and B is nickel, cobalt, manganese, or aluminium. Some cells use higher-capacity negative electrode materials based on AB2 compounds, where A is titanium or vanadium, and B is zirconium or nickel, modified with chromium, cobalt, iron, or manganese. NiMH cells have an alkaline electrolyte, usually potassium hydroxide. The positive electrode is nickel hydroxide, and the negative electrode is hydrogen in the form of an interstitial metal hydride. Hydrophilic polyolefin nonwovens are used for separation. == Charge ==

When fast-charging, it is advisable to charge the NiMH cells with a smart battery charger to avoid overcharging, which can damage cells. Trickle charging The simplest of the safe charging methods is with a fixed low current, with or without a timer. Most manufacturers claim that overcharging is safe at very low currents, below 0.1 C (C/10) (where C is the current equivalent to the capacity of the battery divided by one hour). The Panasonic NiMH charging manual warns that overcharging for long enough can damage a battery and suggests limiting the total charging time to 10–20 hours. NiMH batteries are made of environmentally friendly materials. The batteries contain only mildly toxic substances and are recyclable. Loss of capacity Voltage depression (often mistakenly attributed to the memory effect) from repeated partial discharge can occur, but is reversible with a few full discharge/charge cycles.{{cite web == Discharge ==

A fully charged cell supplies an average 1.25 V/cell during discharge, declining to about 1.0–1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal of the weakest cell). Under a light load (0.5 amperes), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts. Over-discharge Complete discharge of multi-cell packs can cause reverse polarity in one or more cells, which can permanently damage them. This situation can occur in the common arrangement of four AA cells in series, where one cell completely discharges before the others due to small differences in capacity among the cells. When this happens, the good cells start to drive the discharged cell into reverse polarity (i.e. positive anode and negative cathode). Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and perform an auto-shutdown, but devices such as flashlights/torches and some toys do not. Irreversible damage from polarity reversal is a particular danger, even when a low voltage-threshold cutout is employed, when the cells vary in temperature. This is because capacity significantly declines as the cells are cooled. This results in a lower voltage under load of the colder cells. Self-discharge Historically, NiMH cells have had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. The self-discharge rate varies greatly with temperature, where lower storage temperature leads to slower discharge and longer battery life. The self-discharge is on the first day and stabilizes around per day at room temperature. But at it is approximately three times as high. By using improvements to electrode separator, positive electrode, and other components, manufacturers claim the cells retain 70–85% of their capacity when stored for one year at , compared to about half for normal NiMH batteries. They are otherwise similar to standard NiMH batteries, and can be charged in standard NiMH chargers. These cells are marketed as "hybrid", "ready-to-use" or "pre-charged" rechargeables. Retention of charge depends in large part on the battery's resistance to leakage (the higher the better), and on its physical size and charge capacity. Separators keep the two electrodes apart to slow electrical discharge while allowing the transport of ionic charge carriers that close the circuit during the passage of current. High-quality separators are critical for battery performance. The self-discharge rate depends upon separator thickness; thicker separators reduce self-discharge, but also reduce capacity as they leave less space for active components, and thin separators lead to higher self-discharge. Some batteries may have overcome this tradeoff by using more precisely manufactured thin separators, and a sulfonated polyolefin separator, an improvement over the hydrophilic polyolefin based on ethylene vinyl alcohol. Low-self-discharge cells have somewhat lower capacity than otherwise equivalent NiMH cells because of the larger volume of the separator. The highest-capacity low-self-discharge AA cells have 2500 mAh capacity, compared to 2700 mAh for high-capacity AA NiMH cells. Common methods to improve self-discharge include: use of a sulfonated separator (causing removal of N-containing compounds), use of an acrylic acid grafted PP separator (causing reduction in Al- and Mn-debris formation in separator), removal of Co and Mn in A2B7 MH alloy, (causing reduction in debris formation in separator), increase of the amount of electrolyte (causing reduction in the hydrogen diffusion in electrolyte), removal of Cu-containing components (causing reduction in micro-short), PTFE coating on positive electrode (causing suppression of reaction between NiOOH and H2), CMC solution dipping (causing suppression of oxygen evolution), micro-encapsulation of Cu on MH alloy (causing decrease in H2 released from MH alloy), Ni–B alloy coating on MH alloy (causing formation of a protection layer), alkaline treatment of negative electrode (causing reduction of leach-out of Mn and Al), addition of LiOH and NaOH into electrolyte (causing reduction in electrolyte corrosion capabilities), and addition of Al2(SO4)3 into electrolyte (causing reduction in MH alloy corrosion). Most of these improvements have no or negligible effect on cost; some increase cost modestly. == Compared to other battery types ==

Alkaline and other primary batteries NiMH cells are often used in digital cameras and other high-drain devices, where they outperform primary batteries, usually using alkaline chemistry, but also inferior zinc-carbon and chloride. NiMH cells are better for high-current-drain applications than alkaline batteries due to their lower internal resistance. Typical alkaline AA-size batteries, which offer approximately 2.6 Ah capacity at low current demand (25 mA), provide only 1.3 Ah capacity with a 500 mA load. Digital cameras with LCDs and flashlights can draw over 1 A, quickly depleting them. NiMH cells can deliver these current levels without similar loss of capacity. but they were originally significantly more expensive. The cost of lithium batteries fell drastically during the 2010s and many small consumer devices now have non-consumer-replaceable lithium batteries as a result. Lithium batteries produce a higher voltage (3.2–3.7 V nominal), and are thus not a drop-in replacement for AA (alkaline or NiMh) batteries without circuitry to reduce voltage. Although a single lithium cell will typically provide ideal power to replace 3 NiMH cells, the form factor means that the device still needs modification. Lead NiMH batteries can easily be made smaller and lighter than lead-acid batteries and have completely replaced them in small devices. However, lead-acid batteries can deliver huge current at low cost, making lead-acid batteries more suitable for starter motors in combustion vehicles. , nickel–metal hydride batteries constituted three percent of the battery market. == Applications ==

, Japan , Museum Autovision, Altlussheim, Germany Consumer electronics NiMH batteries have replaced NiCd for many roles, notably small rechargeable batteries. NiMH batteries are commonly available in AA (penlight-size) batteries. These have nominal charge capacities (C) of 1.1–2.8 Ah at 1.2 V, measured at the rate that discharges the cell in 5 hours. Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in 1 hour), it does not differ significantly from the nominal capacity. Prior all-electric plug-in vehicles included the General Motors EV1, first-generation Toyota RAV4 EV, Honda EV Plus, Ford Ranger EV and Vectrix scooter. Every first generation hybrid vehicle used NIMH batteries, most notably the Toyota Prius and Honda Insight, as well as later models including the Ford Escape Hybrid, Chevrolet Malibu Hybrid and Honda Civic Hybrid also use them. Patent issues Stanford R. Ovshinsky invented and patented a popular improvement of the NiMH battery and founded Ovonic Battery Company in 1982. General Motors purchased Ovonics' patent in 1994. By the late 1990s, NiMH batteries were being used successfully in many fully electric vehicles, such as the General Motors EV1 and Dodge Caravan EPIC minivan. This generation of electric cars, although successful, was abruptly pulled off the market. In October 2000, the patent was sold to Texaco, and a week later Texaco was acquired by Chevron. Chevron's Cobasys subsidiary provides these batteries only to large OEM orders. General Motors shut down production of the EV1, citing lack of battery availability as a chief obstacle. Cobasys control of NiMH batteries created a patent encumbrance for large automotive NiMH batteries. == See also ==